How Do SPN and FMI Combine to Form a J1939 DTC?

The combination of Suspect Parameter Number (SPN) and Failure Mode Indicator (FMI) is essential for constructing a J1939 Diagnostic Trouble Code (DTC), crucial for diagnosing issues in Mercedes-Benz vehicles; MERCEDES-DIAGNOSTIC-TOOL.EDU.VN provides the resources and expertise to navigate this complex system, making it easier for both professionals and enthusiasts to pinpoint and resolve vehicle malfunctions. Understanding the structure of a J1939 DTC, including SPN, FMI, and diagnostic messages, enhances diagnostic accuracy and repair efficiency, contributing to advanced vehicle diagnostics and maintenance.

1. Understanding J1939 Diagnostic Trouble Codes (DTCs)

J1939 Diagnostic Trouble Codes (DTCs) serve as the foundation for identifying and addressing potential issues within a vehicle’s system. They offer a standardized method for reporting faults, providing crucial information for technicians and vehicle owners.

1.1. What is a J1939 DTC?

A J1939 DTC is a code used to report potential fault conditions in a vehicle’s system. It is a standardized way of communicating diagnostic information between electronic control units (ECUs) in a vehicle network. According to the SAE J1939 standard, a DTC comprises four key components, each providing specific details about the nature and location of the fault. These components are: Suspect Parameter Number (SPN), Failure Mode Identifier (FMI), Occurrence Count (OC), and SPN Conversion Method (CM). Understanding these components is essential for accurate diagnostics and effective repairs.

1.2. The Four Independent Fields of a DTC

A DTC contains four independent fields which give information about the fault being reported. These fields are,

1. Suspect Parameter Number (SPN)
2. Failure Mode Identifier (FMI)
3. Occurrence Count (OC)
4. SPN Conversion Method (CM)

1.3. The Role of SPN

SPN, or Suspect Parameter Number, serves as a unique identifier pinpointing the specific component or subsystem within a vehicle where a fault has been detected; As defined in the SAE J1939 standard, the SPN is a 19-bit number, enabling a vast range of identifiers to accurately specify the location of a problem. SPNs are derived from Parameter Group Numbers (PGNs) already defined by application layer documents, streamlining the diagnostic process by avoiding the need to redefine identifiers. For instance, if a fault is detected in the engine’s fuel injection system, a specific SPN will correspond to that particular component, allowing technicians to quickly locate and address the issue.

1.4. The Role of FMI

FMI, or Failure Mode Identifier, details the exact nature of the detected fault, specifying the type of issue encountered within the identified component or subsystem; According to the SAE J1939 standard, the FMI is a 5-bit number, providing 32 possible failure modes. These modes can indicate electrical system problems, such as short circuits or open circuits, or abnormal conditions, such as data out of range or system failures. For example, an FMI might indicate that a sensor voltage is above normal, a current is below normal, or the system is not responding properly.

1.5. How SPN and FMI Combine

The SPN and FMI work together to provide a comprehensive description of a fault, enabling technicians to quickly understand what component is affected and what type of issue has occurred. The SPN identifies the location of the fault, while the FMI specifies the nature of the fault. This combination allows for targeted diagnostics and efficient repairs.

For example, if the SPN indicates a problem with the engine coolant temperature sensor (location), and the FMI indicates that the voltage is above normal (nature), the technician knows to check the sensor and its wiring for shorts or other electrical issues.

1.6. Understanding Occurrence Count (OC)

Occurrence Count (OC) is a 7-bit number that indicates how many times a particular fault has occurred. Each time a fault transitions from an inactive to an active state, the OC is incremented by 1. However, if the fault becomes active more than 126 times, the OC remains at 126, its maximum value. This parameter is useful for identifying intermittent faults or recurring issues that may require further investigation.

1.7. Understanding SPN Conversion Method (CM)

SPN Conversion Method (CM) is a 1-bit field that indicates the method used for byte alignment in the DTC. Older SAE J1939 specifications supported multiple methods for SPN alignment, while newer versions support only one conversion method, known as Method 4. If the CM bit is set to 1, it means the DTC bytes are aligned using the newer conversion method (Method 4). If the CM bit is 0, it indicates that one of the older conversion methods is used, and the ECU manufacturer should know which of the three methods is used. Method 4 is the recommended method for current implementations.

1.8. Importance of Understanding DTC Components for Mercedes-Benz Vehicles

Understanding the different components of a DTC is crucial for anyone working with Mercedes-Benz vehicles, whether they are professional technicians or vehicle enthusiasts; Mercedes-Benz vehicles often utilize advanced electronic systems, making accurate diagnostics essential for effective repairs and maintenance.

By understanding the SPN, FMI, OC, and CM, technicians can quickly identify the location and nature of a fault, understand how often it has occurred, and correctly interpret the DTC data. This knowledge allows for more efficient troubleshooting, targeted repairs, and ultimately, better vehicle performance and reliability.

2. Detailed Breakdown of SPN (Suspect Parameter Number)

The Suspect Parameter Number (SPN) is a critical component of the J1939 diagnostic system, serving as a unique identifier for specific components or subsystems within a vehicle. It helps technicians pinpoint the exact location of a fault, streamlining the diagnostic process.

2.1. What Does SPN Indicate?

SPN indicates the specific component or subsystem within a vehicle where a fault has been detected. It is a numerical code that corresponds to a particular sensor, actuator, or electronic control unit (ECU). By identifying the SPN, technicians can quickly narrow down the possible causes of a problem and focus their diagnostic efforts on the affected area. For instance, an SPN might indicate a fault in the engine coolant temperature sensor, the fuel injection system, or the transmission control module.

2.2. How is SPN Defined in the J1939 Standard?

In the SAE J1939 standard, the SPN is defined as a 19-bit number. This allows for a wide range of unique identifiers, ensuring that each component or subsystem can be precisely identified. The SPNs are derived from Parameter Group Numbers (PGNs) already defined by application layer documents. This approach streamlines the diagnostic process by reusing existing identifiers rather than creating new ones. The standard also specifies how SPNs are used in diagnostic messages and how they relate to other diagnostic parameters, such as FMIs.

2.3. Examples of Common SPNs in Mercedes-Benz Vehicles

Mercedes-Benz vehicles utilize a variety of SPNs to identify different components and subsystems. Here are a few examples of common SPNs and their corresponding components:

SPN	Component/Subsystem	Description
100	Engine Coolant Temperature Sensor	Indicates a fault with the engine coolant temperature sensor or its circuit.
102	Intake Manifold Pressure Sensor	Indicates a fault with the intake manifold pressure sensor or its circuit.
108	Atmospheric Pressure Sensor	Indicates a fault with the atmospheric pressure sensor or its circuit.
597	Engine Fuel Rail Pressure Sensor	Indicates a fault with the engine fuel rail pressure sensor or its circuit.
651	Cylinder 1 Injector	Indicates a fault with the fuel injector for cylinder 1.
723	Vehicle Speed Sensor	Indicates a fault with the vehicle speed sensor or its circuit.
1079	Transmission Output Speed Sensor	Indicates a fault with the transmission output speed sensor or its circuit.
3031	SCR Catalyst Temperature Sensor	Indicates a fault with the Selective Catalytic Reduction (SCR) catalyst temperature sensor or circuit.
3364	Diesel Particulate Filter (DPF)	Indicates a fault with the Diesel Particulate Filter (DPF) system.

These are just a few examples, and the specific SPNs used in a Mercedes-Benz vehicle will depend on the model, year, and engine type. Technicians can refer to service manuals or diagnostic databases to find the SPNs relevant to a particular vehicle.

2.4. Resources for Identifying SPNs

Several resources are available to help technicians identify SPNs and their corresponding components:

• Service Manuals: Mercedes-Benz service manuals provide detailed information on diagnostic codes, including SPNs, for specific vehicle models.
• Diagnostic Databases: Online diagnostic databases, such as those offered by MERCEDES-DIAGNOSTIC-TOOL.EDU.VN, allow technicians to search for SPNs and access information on their meaning, potential causes, and troubleshooting steps.
• Diagnostic Tools: Advanced diagnostic tools can automatically decode SPNs and provide information on the affected component.

2.5. Using SPN for Efficient Diagnostics

By accurately identifying the SPN, technicians can significantly reduce diagnostic time and improve the accuracy of their repairs. When a DTC is triggered, the SPN provides a clear indication of where to start troubleshooting. This targeted approach avoids the need to perform unnecessary tests and inspections, saving time and resources.

3. Deep Dive into FMI (Failure Mode Identifier)

The Failure Mode Identifier (FMI) is another essential component of the J1939 diagnostic system. It provides specific information about the nature of the fault, detailing the type of issue encountered within the component or subsystem identified by the SPN.

3.1. What Does FMI Tell You About the Fault?

FMI specifies the nature of the fault detected by DTC. It describes what type of issue has occurred within the component or subsystem identified by the SPN. The FMI can indicate a wide range of problems, including electrical issues, data errors, mechanical failures, and system malfunctions.

For example, an FMI might indicate that a sensor voltage is too high or too low, a current is outside of its normal range, a signal is erratic, or the system is not responding properly. By understanding the FMI, technicians can gain valuable insight into the root cause of the problem and develop an effective repair strategy.

3.2. Standard FMI Definitions and Their Meanings

The SAE J1939 standard defines a set of FMI values, each with a specific meaning. The FMI is a 5-bit number, providing 32 possible failure modes. Here is a list of the standard FMI definitions and their meanings:

FMI	Description	Possible Causes
0	High – most severe (3)	Signal is higher than expected. Could be due to a short to power, a faulty sensor, or a wiring issue.
1	Low – most severe (3)	Signal is lower than expected. Could be due to a short to ground, a faulty sensor, or a wiring issue.
2	Erratic, Intermittent, or Incorrect	Signal is unstable or inconsistent. Could be due to a loose connection, a faulty sensor, or electrical interference.
3	Voltage Above Normal or shorted to high fault	Voltage is higher than the expected range. Could be due to a short to power or a faulty voltage regulator.
4	Voltage Below Normal	Voltage is lower than the expected range. Could be due to a short to ground or a faulty voltage regulator.
5	Current Below Normal or open circuit fault	Current is lower than the expected range. Could be due to an open circuit, a faulty sensor, or a wiring issue.
6	Current Above Normal or Shorted to ground fault	Current is higher than the expected range. Could be due to a short to ground, a faulty actuator, or a wiring issue.
7	System Not Responding Properly	The system is not responding to commands or requests. Could be due to a faulty ECU, a communication error, or a software issue.
8	Abnormal Frequency, Pulse Width, or Period	The frequency, pulse width, or period of a signal is outside the expected range. Could be due to a faulty sensor, a faulty actuator, or a timing issue.
9	Abnormal Update Rate	The data is not being updated at the expected rate. Could be due to a communication error, a faulty sensor, or a software issue.
10	Abnormal Rate of Change	The rate of change of the data is outside the expected range. Could be due to a faulty sensor, a faulty actuator, or a system instability.
11	Other Failure Mode	The failure mode does not fit into any of the other categories. This FMI is used for non-standard or manufacturer-specific failure modes.
12	Failure	Indicates a general failure of the component or subsystem. This FMI is often used when the specific nature of the failure is unknown.
13	Out of Calibration	The component or subsystem is out of calibration. This could be due to wear, damage, or improper adjustment.
14	Special Instruction	This FMI is used to indicate that special instructions or procedures are required to diagnose or repair the fault.
15	data valid but above normal range– least severe	The data is valid but above the normal operating range. This FMI is used for minor deviations from the normal range.
16	data valid but above normal range – moderate severity	The data is valid but above the normal operating range. This FMI is used for more significant deviations from the normal range.
17	data valid but below normal range – least severe	The data is valid but below the normal operating range. This FMI is used for minor deviations from the normal range.
18	data valid but below normal range – moderate severity	The data is valid but below the normal operating range. This FMI is used for more significant deviations from the normal range.
19	Received network Data Error	There is an error in the data received from the network. This could be due to a communication error, a faulty ECU, or a software issue.
20	Data Drifted High	The data has drifted above the expected range over time. This could be due to sensor degradation, component aging, or environmental factors.
21	Data Drifted Low	The data has drifted below the expected range over time. This could be due to sensor degradation, component aging, or environmental factors.
31	Condition exists	Indicates that a specific condition exists within the system. This FMI is often used for diagnostic or monitoring purposes.

3.3. Examples of FMIs in Diagnosing Mercedes-Benz Issues

Here are some examples of how FMIs can be used to diagnose issues in Mercedes-Benz vehicles:

• SPN 100 (Engine Coolant Temperature Sensor) with FMI 3 (Voltage Above Normal): This indicates that the voltage signal from the engine coolant temperature sensor is higher than expected. This could be due to a short to power in the sensor circuit.
• SPN 102 (Intake Manifold Pressure Sensor) with FMI 4 (Voltage Below Normal): This indicates that the voltage signal from the intake manifold pressure sensor is lower than expected. This could be due to a short to ground in the sensor circuit.
• SPN 651 (Cylinder 1 Injector) with FMI 5 (Current Below Normal): This indicates that the current flowing through the fuel injector for cylinder 1 is lower than expected. This could be due to an open circuit in the injector or its wiring.
• SPN 723 (Vehicle Speed Sensor) with FMI 2 (Erratic, Intermittent, or Incorrect): This indicates that the signal from the vehicle speed sensor is unstable or inconsistent. This could be due to a loose connection or a faulty sensor.

3.4. Utilizing FMI for Targeted Troubleshooting

By understanding the FMI, technicians can develop a targeted troubleshooting strategy. The FMI provides valuable information about the type of issue that has occurred, allowing technicians to focus their efforts on the most likely causes. This approach saves time and resources, and it increases the likelihood of a successful repair.

For example, if an FMI indicates a voltage above normal, the technician knows to check the sensor circuit for shorts to power. If the FMI indicates a current below normal, the technician knows to check for open circuits or faulty components.

3.5. Resources for FMI Information

Several resources are available to help technicians understand FMI definitions and their implications:

• SAE J1939 Standard: The SAE J1939 standard provides detailed definitions of all FMI values.
• Service Manuals: Mercedes-Benz service manuals provide information on diagnostic codes, including FMIs, for specific vehicle models.
• Diagnostic Databases: Online diagnostic databases, such as those offered by MERCEDES-DIAGNOSTIC-TOOL.EDU.VN, allow technicians to search for FMIs and access information on their meaning, potential causes, and troubleshooting steps.
• Diagnostic Tools: Advanced diagnostic tools can automatically decode FMIs and provide information on the type of fault that has occurred.

4. Putting It All Together: Real-World Examples

To illustrate how SPN and FMI work together in real-world scenarios, let’s examine some practical examples of diagnosing issues in Mercedes-Benz vehicles.

4.1. Example 1: Engine Coolant Temperature Sensor Issue

Scenario: A Mercedes-Benz vehicle displays a check engine light, and a diagnostic scan reveals the following DTC:

• SPN: 100 (Engine Coolant Temperature Sensor)
• FMI: 3 (Voltage Above Normal)

Analysis:

• The SPN indicates that the issue is related to the engine coolant temperature sensor.
• The FMI indicates that the voltage signal from the sensor is higher than expected.

Troubleshooting Steps:

1. Check the wiring and connections to the engine coolant temperature sensor for shorts to power.
2. Inspect the sensor for damage or corrosion.
3. Use a multimeter to measure the voltage signal from the sensor.
4. If the wiring and sensor appear to be in good condition, replace the engine coolant temperature sensor.

4.2. Example 2: Intake Manifold Pressure Sensor Issue

Scenario: A Mercedes-Benz vehicle is experiencing a loss of power, and a diagnostic scan reveals the following DTC:

• SPN: 102 (Intake Manifold Pressure Sensor)
• FMI: 4 (Voltage Below Normal)

Analysis:

• The SPN indicates that the issue is related to the intake manifold pressure sensor.
• The FMI indicates that the voltage signal from the sensor is lower than expected.

Troubleshooting Steps:

1. Check the wiring and connections to the intake manifold pressure sensor for shorts to ground.
2. Inspect the sensor for damage or contamination.
3. Use a multimeter to measure the voltage signal from the sensor.
4. Check the vacuum lines connected to the sensor for leaks or damage.
5. If the wiring, sensor, and vacuum lines appear to be in good condition, replace the intake manifold pressure sensor.

4.3. Example 3: Cylinder 1 Injector Issue

Scenario: A Mercedes-Benz vehicle is misfiring, and a diagnostic scan reveals the following DTC:

• SPN: 651 (Cylinder 1 Injector)
• FMI: 5 (Current Below Normal)

Analysis:

• The SPN indicates that the issue is related to the fuel injector for cylinder 1.
• The FMI indicates that the current flowing through the injector is lower than expected.

Troubleshooting Steps:

1. Check the wiring and connections to the cylinder 1 injector for open circuits or damage.
2. Use a multimeter to measure the resistance of the injector.
3. Check the fuel supply to the injector.
4. If the wiring, injector, and fuel supply appear to be in good condition, replace the cylinder 1 injector.

4.4. Example 4: Vehicle Speed Sensor Issue

Scenario: A Mercedes-Benz vehicle’s speedometer is not working correctly, and a diagnostic scan reveals the following DTC:

• SPN: 723 (Vehicle Speed Sensor)
• FMI: 2 (Erratic, Intermittent, or Incorrect)

Analysis:

• The SPN indicates that the issue is related to the vehicle speed sensor.
• The FMI indicates that the signal from the sensor is unstable or inconsistent.

Troubleshooting Steps:

1. Check the wiring and connections to the vehicle speed sensor for looseness or damage.
2. Inspect the sensor for damage or contamination.
3. Use an oscilloscope to monitor the signal from the sensor while the vehicle is moving.
4. If the wiring and sensor appear to be in good condition, replace the vehicle speed sensor.

4.5. The Importance of Accurate Diagnosis

These examples demonstrate how the combination of SPN and FMI can provide valuable information for diagnosing issues in Mercedes-Benz vehicles. By accurately identifying the SPN and FMI, technicians can quickly narrow down the possible causes of a problem and develop an effective repair strategy. This targeted approach saves time and resources, and it increases the likelihood of a successful repair.

5. Diagnostic Messages in J1939

Diagnostic Messages (DM) play a crucial role in the J1939 standard, facilitating the exchange of diagnostic information between networked ECUs in a vehicle. These messages are used both during vehicle repair and during normal vehicle operation, enabling intelligent systems to self-adjust in the presence of faults or compensate based on the diagnostic information.

5.1. Overview of Diagnostic Messages (DM)

Diagnostic messages are standardized messages defined in the SAE J1939 standard for transmitting diagnostic information between ECUs. These messages contain information about active and previously active DTCs, as well as other diagnostic data such as freeze frame parameters and lamp status. By using diagnostic messages, ECUs can communicate faults and other diagnostic information to each other, allowing for a more comprehensive understanding of the vehicle’s health.

5.2. Common Diagnostic Messages: DM1, DM2, DM3, DM4, DM11, DM12

The J1939 standard defines several diagnostic messages, each with a specific purpose. Here are some of the most common diagnostic messages:

• DM1 (Diagnostic Message 1): Contains information about all active DTCs and diagnostic lamp status. This message is sent periodically when there is an active DTC or in response to a request.
• DM2 (Diagnostic Message 2): Contains a list of previously active DTCs. This message is sent only on request.
• DM3 (Diagnostic Message 3): Used to clear all diagnostic information related to previously active DTCs.
• DM4 (Diagnostic Message 4): Used to read freeze frame data, which contains recorded data corresponding to a DTC when the fault has occurred.
• DM11 (Diagnostic Message 11): Used to clear diagnostic data for active DTCs.
• DM12 (Diagnostic Message 12): Sends information of active DTCs only related to emission.

5.3. DM1: Active Diagnostics Trouble Codes

DM1 message contains information of all active DTCs and diagnostic lamp status. The lamp status supports visual diagnostics and used by fault indicators on a vehicle dashboard. SAE J1939 diagnostic supports four types of lamp status. Malfunction Indicator Lamp (MIL) shows malfunctions related to emission. Red Stop Lamp (RSL) indicates serious faults that require the vehicle to stop. Amber Warning Lamp (AWL) signals less critical faults and vehicles need not be immediately stopped. The vehicle can still run while these faults are active. Protection Lamp (PL) indicates faults that are not because of electronics like hydraulic fluid temperature is rising beyond prescribed temperature range. All these lamp status supports 4 states; Lamp OFF, Lamp Steady ON, Lamp Flashing 1 Hz, Lamp flashing 2 Hz. if ECU does not have an active fault, them lamp status shall be set to Lamp Off.

DM1 message uses PGN 65226 (0xFECA). DM1 message is sent periodically only when there is an active DTC or in the response of the request. If there is more than 1 active DTC, this message is sent using the transport protocol. The transmission rate of this message is 1 second. When a DTC becomes active DM1 message is sent immediately and then every one second thereafter. If a different DTC changes state (inactive to active or active to inactive) within the 1 second update period, a DM1 message is transmitted to communicate the change. To avoid high transmission rates, it is recommended that only one state change of one DTC shall be transmitted in one second. For a DTC becoming active and inactive twice within the 1-second interval, only one DM1 message is sent in the one-second interval. The following table shows the DM1 message format.

Default Priority	6
PDU Format	254
PDU Specific	202
PGN	65226
Byte 1 – bits 8-7	Malfunction Indicator Lamp status
Byte 1 -bits 6-5	Red Stop Lamp status
Byte 1 -bits 4-3	Amber Warning Lamp status
Byte 1 -bits 2-1	Protect Lamp status
Byte 2 – bits 8-7	Reserved
Byte 2 -bits 6-5	Reserved
Byte 2 -bits 4-3	Reserved
Byte 2 -bits 2-1	Reserved
Byte 3 – bits 8-1	SPN
Byte 4 – bits 8-1	SPN
Byte 5 – bits 8-6	SPN
Byte 5 -bits 5-1	FMI
Byte 6 – bit 8	SPN Conversion Method (CM)
Byte 6 -bit 7-1	Occurance Count (OC)

5.4. DM2: Previously Active Diagnostics Trouble Codes

DM2 messages contain a list of previously active DTCs. This message contains all the DTCs which were previously active and the occurrence count is non zero. Like DM1 message if the number of DTCs is more than 1, DM2 message is sent using the transport protocol. This message is not a periodic message, so it is sent only on request using PGN 59904. If this PGN is not supported then a NACK response shall be sent. DM2 message uses 65227 (0xFECB). The following table shows DM2 message details.

Default Priority	6
PDU Format	254
PDU Specific	203
PGN	65227
Byte 1 – bits 8-7	Malfunction Indicator Lamp status
Byte 1 – bits 6-5	Red Stop Lamp status
Byte 1 – bits 4-3	Amber Warning Lamp status
Byte 1 – bits 2-1	Protect Lamp status
Byte 2 – bits 8-7	Reserved
Byte 2 – bits 6-5	Reserved
Byte 2 – bits 4-3	Reserved
Byte 2 – bits 2-1	Reserved
Byte 3 – bits 8-1	SPN
Byte 4 – bits 8-1	SPN
Byte 5 – bits 8-6	SPN
Byte 5 -bits 5-1	FMI
Byte 6 – bit 8	SPN Conversion Method (CM)
Byte 6 -bit 7-1	Occurance Count (OC)

5.5. DM3: Diagnostics Data Clear of Previously Active DTCs

DM3 message is used to clear all diagnostic information related to previously active DTCs. This information may include the following items.

• Number of previously active DTCs
• Previously active DTCs
• Freeze frame data
• Status of system monitoring tests
• On-Board monitoring tests
• Distance traveled while MIL (malfunction indicator lamp) is active
• Performance monitoring information
• manufacturer specific diagnostic data

DM3 message is sent using request PGN 59904. Once the previously active diagnostic data is clear or there is no previously active DTC data, the positive acknowledgment is sent by the controller. If the diagnostic data clear operation is failed or the controller could not perform the operation for some reason, then a negative acknowledgment is sent by the controller.

DM3 message does not affect active DTC data. The below table shows DM3 message details.

Default Priority	6
PDU Format	254
PDU Specific	204
PGN	65228
Byte 1 – 8	No data for this message. All bytes are set to FF.

5.6. DM4: Freeze Frame Parameters

DM4 message (PGN 65229) is used to read freeze frame data. The freeze-frame contains recorded data corresponding to a DTC when the fault has occurred. A freeze-frame is associated with only DTC and a DTC can have only one freeze-frame. The maximum number of bytes in a freeze frame data can be 1785 bytes so that it can fit into the TP frame. DM4 message is not a periodic message and requested using PGN 59904. The controller shall send NACK if the DM4 message is not supported.

5.7. DM11: Diagnostics Data Clear of Active DTCs

DM11 messages used PGN 65235 (0xFED3), This message is used to clear diagnostic data for active DTCs. Working on this message is exactly the same as the DM3 message.

Below table shows DM11 message details.

Default Priority	6
PDU Format	254
PDU Specific	211
PGN	65235
Byte 1 – Byte 8	No data for this message. All bytes are set to FF.

5.8. DM12: Emission Related Active DTCs

DM12 messages send information of active DTCs only related to emission. This message contains lamp status and a list of DTCs. if there are more than 1 DTC then transport protocol is used to transmit this message. This message is not periodic and sent only when requested using request PGN 59904. The following table shows DM12 message format.

Default Priority	6
PDU Format	254
PDU Specific	212
PGN	65236
Byte 1 – bits 8-7	Malfunction Indicator Lamp status
Byte 1 -bits 6-5	Red Stop Lamp status
Byte 1 -bits 4-3	Amber Warning Lamp status
Byte 1 -bits 2-1	Protect Lamp status
Byte 2 – bits 8-7	Reserved
Byte 2 – bits 6-5	Reserved
Byte 2 – bits 4-3	Reserved
Byte 2 – bits 2-1	Reserved
Byte 3 – bits 8-1	SPN
Byte 4 – bits 8-1	SPN
Byte 5 – bits 8-6	SPN
Byte 5 -bits 5-1	FMI
Byte 6 – bit 8	SPN Conversion Method (CM)
Byte 6 -bit 7-1	Occurance Count (OC)

5.9. How Diagnostic Messages Aid in Troubleshooting

Diagnostic messages provide a wealth of information that can be used to troubleshoot issues in Mercedes-Benz vehicles. By reading and interpreting these messages, technicians can:

• Identify active and previously active DTCs
• Determine the nature of the fault (using the FMI)
• Access freeze frame data, which provides a snapshot of the vehicle’s operating conditions when the fault occurred
• Clear DTCs after repairs have been completed

5.10. Tools for Reading and Interpreting Diagnostic Messages

To effectively utilize diagnostic messages, technicians need specialized tools that can read and interpret the data. These tools include:

• Diagnostic Scanners: These handheld devices can connect to the vehicle’s diagnostic port and read diagnostic messages.
• PC-Based Diagnostic Software: Software programs that run on a computer and connect to the vehicle’s diagnostic port via an interface cable. These programs often offer more advanced features than handheld scanners.
• MERCEDES-DIAGNOSTIC-TOOL.EDU.VN: Provides resources and expertise to help technicians understand and interpret diagnostic messages in Mercedes-Benz vehicles.

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• Diagnostic message guides: Information on the different types of diagnostic messages, their structure, and their interpretation.
• Troubleshooting guides: Step-by-step instructions for diagnosing and repairing common issues in Mercedes-Benz vehicles.
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