Can You Reset Monitors For OBD2 To Pass Smog Check?

Yes, you can reset OBD2 monitors, but it’s not always a straightforward solution for passing a smog check, MERCEDES-DIAGNOSTIC-TOOL.EDU.VN explains the intricacies of OBD2 monitor resetting and its implications for smog inspections. Understanding the process and potential issues is essential for vehicle owners and technicians alike. This article delves into the details of resetting OBD2 monitors, smog check standards, and how to address common challenges. Learn about OBD systems, emission control functionality, and diagnostic trouble codes.

1. What Are OBD2 Monitors and Why Do They Matter for Smog Checks?

OBD2 (On-Board Diagnostics II) monitors are self-tests performed by your vehicle’s computer to ensure its emission control systems are functioning correctly. These monitors assess various components like the catalytic converter, oxygen sensors, evaporative system, and more.

Why They Matter: Smog checks, also known as emissions tests, are required in many states to ensure vehicles meet specific air quality standards. During a smog check, the OBD2 system is scanned to verify that all required monitors have completed and are reporting “ready” status. If monitors are incomplete or show a fault, your vehicle may fail the smog check. Incomplete monitors can indicate that the vehicle hasn’t been driven enough to complete the necessary self-tests or that there might be underlying issues preventing the monitors from running.

2. Understanding OBD2 Readiness Monitors: A Deep Dive

Readiness monitors are crucial for ensuring your vehicle passes a smog check. Let’s break down what they are and why they are important:

2.1. What are Readiness Monitors?

Readiness monitors are diagnostic tests run by your car’s onboard computer (ECU or PCM) to verify the functionality of various emission control systems. These tests confirm that components like the catalytic converter, oxygen sensors, EGR (Exhaust Gas Recirculation) system, evaporative emissions control system, and others are working as intended. Each monitor has specific criteria that must be met for the test to run and complete successfully.

2.2. Types of Readiness Monitors

There are several types of readiness monitors, each focusing on a different aspect of your vehicle’s emissions system:

• Catalyst Monitor: Checks the efficiency of the catalytic converter in reducing harmful emissions.
• Oxygen Sensor Monitor: Verifies that the oxygen sensors are functioning correctly and providing accurate data to the ECU.
• EGR System Monitor: Tests the operation of the EGR valve and its ability to reduce NOx emissions.
• Evaporative System Monitor: Checks for leaks in the fuel vapor recovery system.
• Secondary Air System Monitor: (If equipped) Tests the secondary air injection system, which helps to reduce cold start emissions.
• Heated Catalyst Monitor: (If equipped) Checks the performance of the heated catalyst.
• Oxygen Sensor Heater Monitor: Verifies the functionality of the oxygen sensor heaters, which help the sensors reach operating temperature quickly.
• Misfire Monitor: Detects engine misfires that can lead to increased emissions and engine damage.
• Fuel System Monitor: Checks the fuel delivery system for proper operation.
• Comprehensive Components Monitor: Checks various other components and systems for proper function.
• NMHC Catalyst Monitor: (Diesel vehicles) Checks the efficiency of the Non-Methane Hydrocarbon catalyst.
• NOx Aftertreatment Monitor: (Diesel vehicles) Verifies the operation of the NOx reduction system.
• Particulate Filter Monitor: (Diesel vehicles) Checks the performance of the diesel particulate filter (DPF) in trapping soot.
• Exhaust Gas Sensor Monitor: (Diesel vehicles) Monitors the performance of the exhaust gas sensors.

2.3. Factors Affecting Monitor Readiness

Several factors can influence whether or not a readiness monitor will run and complete:

• Drive Cycle: Each monitor has a specific drive cycle or set of driving conditions that must be met for the test to initiate and complete. This typically involves a combination of highway and city driving, specific speeds, and engine load conditions.
• Vehicle Condition: Any underlying mechanical or electrical issues can prevent monitors from running. For example, a faulty oxygen sensor will prevent the oxygen sensor monitor from completing.
• Battery Disconnection: Disconnecting the vehicle’s battery will reset all readiness monitors to an incomplete state.
• Diagnostic Trouble Codes (DTCs): The presence of certain DTCs can prevent related monitors from running.
• Environmental Conditions: Some monitors may not run if the ambient temperature is too high or too low.
• Fuel Level: Some evaporative system monitors require a specific fuel level to run.

2.4. Readiness Monitor Status and Smog Checks

During a smog check, the testing equipment reads the status of the readiness monitors from your vehicle’s computer. The number of incomplete monitors allowed varies by model year and fuel type:

Model Year	Fuel Type	Number of Incomplete Monitors Allowed to Pass OBD Test
1996-1999	Gas	Any one
2000 and newer	Gas	Evaporative system
1998-2006	Diesel	Zero
2007 and newer	Diesel	Particulate Filter and Non-Methane Hydrocarbon (NMHC)

If your vehicle exceeds the allowed number of incomplete monitors, it will fail the smog check.

2.5. Troubleshooting Incomplete Monitors

If you find that your vehicle has incomplete monitors, here are some steps you can take:

1. Consult Your Owner’s Manual: Check your owner’s manual for specific drive cycle recommendations for your vehicle.
2. Perform a General Drive Cycle: A general drive cycle typically involves a combination of city and highway driving, with variations in speed and acceleration.
3. Address Any Underlying Issues: If you suspect any mechanical or electrical problems, have your vehicle diagnosed and repaired by a qualified mechanic.
4. Use a Scan Tool: A scan tool can help you monitor the status of the readiness monitors and identify any DTCs that may be preventing them from running.
5. Give it Time: Sometimes, it simply takes time and repeated driving cycles for the monitors to complete.
6. Consider a Professional: If you are unable to resolve the issue yourself, consult a qualified mechanic who specializes in emissions diagnostics.

2.6. Case Study: Ford Transit Exhaust Gas Sensor Monitor Issue

Problem: Some 2015-2019 Ford Transit 3.2L diesel vehicles fail smog checks because the Exhaust Gas Sensor (EGS) monitor will not complete.

Cause: After a Diesel Oxidation Catalyst (DOC) or Diesel Particulate Filter (DPF) replacement, the EGS monitor will not run until a “degreening strategy” completes, which can take 6,000 to 10,000 miles.

Solution:

• Testing Procedure: During the smog check, the OIS (On-Board Inspection System) will ignore the incomplete EGS monitor for specific VINs where the DOC or DPF were replaced after January 1, 2022.
• Expiration: This bypass is valid until October 8, 2025.

Key Takeaway:

• Technical Service Bulletins (TSBs): Always consult TSBs and recall information for known issues and specific testing procedures.
• OIS Programming: Smog check systems like OIS are often programmed to accommodate known vehicle-specific issues.
• VIN-Specific Information: Solutions may be VIN-specific, so accurate vehicle identification is crucial.
• Temporary Bypasses: Some solutions are temporary, with expiration dates, requiring ongoing awareness of updates and changes.

3. How to Reset OBD2 Monitors

Resetting OBD2 monitors involves clearing the vehicle’s computer memory of any stored diagnostic trouble codes (DTCs) and readiness monitor status.

3.1. Using a Scan Tool

A scan tool is the most common method for resetting OBD2 monitors.

1. Connect the Scan Tool: Plug the scan tool into the OBD2 port, usually located under the dashboard on the driver’s side.
2. Power On: Turn the ignition key to the “on” position without starting the engine.
3. Navigate the Menu: Follow the scan tool’s instructions to navigate to the “Diagnostic Menu” or similar option.
4. Clear Codes: Select the “Clear Codes” or “Erase Codes” function. This will erase any stored DTCs and reset the readiness monitors.
5. Verify Reset: After clearing the codes, verify that the monitors have been reset to “not ready” or “incomplete” status.

3.2. Disconnecting the Battery

Disconnecting the battery can also reset the OBD2 system, but it’s not the recommended method.

1. Disconnect the Negative Terminal: Disconnect the negative battery terminal using a wrench.
2. Wait: Wait for 15-30 minutes to allow the vehicle’s computer to fully reset.
3. Reconnect the Terminal: Reconnect the negative battery terminal.

Important Considerations:

• Data Loss: Disconnecting the battery will erase all stored data, including radio presets, seat positions, and other customized settings.
• Potential Damage: Improperly disconnecting or reconnecting the battery can damage the vehicle’s electrical system.

3.3. Why You Shouldn’t Just Reset Monitors to Pass a Smog Check

While resetting the monitors might seem like a quick fix, it’s essential to understand the consequences. Smog check technicians are trained to identify vehicles that have recently had their OBD2 systems reset. If the monitors haven’t had enough time to run and complete, it will raise a red flag, and your vehicle will likely fail the test.

Reason: Resetting the OBD-II system clears all diagnostic trouble codes (DTCs) and resets the readiness monitors. While this might temporarily remove the check engine light, it also means that your vehicle hasn’t confirmed whether the underlying issues that triggered the codes have been resolved. Resetting without proper verification or repair is seen as an attempt to bypass the emissions testing system.

4. The Importance of Drive Cycles After Resetting

After resetting the OBD2 monitors, you’ll need to perform a drive cycle to allow the monitors to run and complete. A drive cycle is a specific set of driving conditions that must be met to initiate the self-tests.

4.1. What is a Drive Cycle?

A drive cycle is a series of driving maneuvers designed to simulate real-world driving conditions and allow the OBD2 monitors to run their self-tests. Each monitor has specific criteria that must be met for the test to initiate and complete successfully.

4.2. General Drive Cycle Procedure

While the exact drive cycle procedure varies by vehicle make and model, a general drive cycle typically includes the following steps:

1. Cold Start: Start the engine when it’s cold (after sitting for several hours).
2. Idle: Let the engine idle for a few minutes.
3. Acceleration: Accelerate smoothly to a moderate speed (e.g., 45 mph).
4. Cruise: Maintain a steady speed for several minutes.
5. Deceleration: Decelerate gradually without using the brakes.
6. Stop: Come to a complete stop.
7. Repeat: Repeat the above steps several times.

4.3. Vehicle-Specific Drive Cycles

For the most accurate and effective drive cycle, consult your vehicle’s owner’s manual or a reliable repair manual. These resources will provide detailed instructions specific to your vehicle’s make, model, and engine.

4.4. Monitoring Progress with a Scan Tool

A scan tool can be invaluable for monitoring the progress of the drive cycle. Connect the scan tool to your vehicle’s OBD2 port and use it to check the status of the readiness monitors. The scan tool will indicate which monitors have completed and which ones are still incomplete.

4.5. Case Study: BMW Oxygen Sensor Monitor

Problem: Some BMWs may fail inspection due to the oxygen sensor readiness monitor not setting.

Cause: This is often due to a DME (Digital Motor Electronics) software error.

Solution:

• Check DME Software: Verify the DME “programmed control unit” number listed in BMW Technical Service Bulletin (TSB) #SI B12 16 16.
• Reprogram DME: If the software version is outdated, reprogram the DME with the updated software as per the TSB instructions.

Key Takeaway:

• TSBs are Essential: Always check for relevant TSBs for specific makes and models. TSBs provide valuable information on known issues and their solutions.
• Software Updates: In many cases, emission-related issues can be resolved with software updates to the vehicle’s computer.
• Proper Identification: Accurate identification of the vehicle and its software version is crucial for applying the correct fix.

5. Common Reasons for OBD2 Monitors Not Resetting

Even after performing a drive cycle, some OBD2 monitors may still refuse to reset. Here are some common reasons why:

5.1. Underlying Mechanical Issues

If there are underlying mechanical or electrical issues, the monitors may not run or complete. For example, a faulty oxygen sensor, a vacuum leak, or a malfunctioning EGR valve can all prevent the monitors from resetting. In such cases, addressing the root cause of the problem is essential before attempting to reset the monitors.

5.2. Faulty Sensors

Faulty sensors are a common culprit behind persistent monitor issues. Sensors like oxygen sensors, mass airflow sensors, and coolant temperature sensors play a critical role in the operation of the emission control systems. If these sensors are providing inaccurate data, the monitors may not run correctly.

5.3. Wiring Problems

Wiring problems, such as damaged or corroded wires, can also interfere with the operation of the OBD2 system. Check the wiring and connectors associated with the emission control components for any signs of damage.

5.4. Software Glitches

In some cases, software glitches in the vehicle’s computer can prevent the monitors from resetting. This is especially common in older vehicles. A software update or reflash may be necessary to resolve the issue.

5.5. Case Study: Chevrolet Express Exhaust Gas Sensor Monitor Issue

Problem: The Exhaust Gas Sensor monitor will not complete on some 2019 Chevrolet Express 2.8L diesel vehicles. Additionally, a Particulate Matter sensor fault may not set a DTC or illuminate the MIL (Malfunction Indicator Lamp).

Cause: The Engine Control Module (ECM) may not properly handle diagnostic information for the exhaust Particulate Matter (PM) sensor.

Solution:

• ECM Reprogramming: Dealers are instructed to reprogram the ECM per Emission Recall N232408230.
• Testing Procedure: After performing the recall, retest the vehicle to ensure the monitor completes and the PM sensor fault is correctly diagnosed.

Key Takeaway:

• Emission Recalls: Stay informed about emission recalls, as they often address software or hardware issues affecting monitor readiness.
• Proper Diagnostics: Ensure that faults are correctly diagnosed and that the ECM is handling diagnostic information as intended.
• Retesting: Always retest after applying a fix to verify that the issue has been resolved and the monitors are completing.

6. Specific Vehicle Issues and Solutions

Certain vehicle makes and models are known to have specific issues that can prevent OBD2 monitors from resetting. Here are a few examples:

6.1. Ford Vehicles

Some Ford vehicles, especially those with flex-fuel engines, may have difficulty completing the evaporative system monitor. The issue is due to the system monitors being disabled for operation with high ethanol blends. OIS (On-Board Inspection System) ignores all 11 readiness monitors on these vehicles.

6.2. BMW Vehicles

Certain BMW models may not pass inspection due to the oxygen sensors readiness monitor not setting. This is often due to a DME (Digital Motor Electronics) software error. Reprogramming the DME with the updated software may resolve the issue.

6.3. GM Vehicles

Some GM vehicles may have a message appear during the OBD II test stating, “Unable to confirm this Vehicle’s OBD II requirements. Does this vehicle have OBD II?” This is because federally certified vehicles may not be certified to OBD II standards. Check the under-hood emission label to determine the OBD certification status.

6.4. Case Study: Jaguar/Land Rover NOx Aftertreatment Monitor Issue

Problem: Certain Jaguar and Land Rover diesel models (specifically 2.0L engines from 2017-2019) fail the OBD test because the NOx Aftertreatment System monitor will not complete.

Cause: This issue is attributed to a vehicle design anomaly.

Solution:

• Dealer Recall: The manufacturer has issued a recall (e.g., H444 for Jaguar, N808 for Land Rover) to address this issue.
• Software Update: Direct the vehicle owner to a Jaguar or Land Rover dealership for a software update to resolve the problem.
• Testing Procedure: During the smog check, the OIS will detect this issue and prompt the inspector to advise the owner to seek the software update.

Key Takeaway:

• Vehicle-Specific Issues: Some monitor issues are specific to certain makes and models due to design anomalies or software problems.
• Manufacturer Involvement: Manufacturers often release recalls or technical service bulletins (TSBs) to address these issues.
• Software Updates: The solution frequently involves updating the vehicle’s software to correct the anomaly.
• Communication is Key: Smog check inspectors should be aware of these issues and advise vehicle owners to seek appropriate service.

7. When to Seek Professional Help

If you’ve tried resetting the OBD2 monitors and performing a drive cycle, but the monitors still won’t reset, it’s time to seek professional help. A qualified mechanic can diagnose the underlying cause of the problem and recommend the appropriate repairs.

7.1. Diagnosing Complex Issues

Diagnosing OBD2 monitor issues can be complex, requiring specialized tools and knowledge. A mechanic can use a scan tool to read diagnostic trouble codes (DTCs), monitor sensor data, and perform other tests to pinpoint the source of the problem.

7.2. Performing Repairs

Once the problem has been diagnosed, a mechanic can perform the necessary repairs. This may involve replacing faulty sensors, repairing wiring problems, or performing a software update.

7.3. Ensuring Smog Check Compliance

A mechanic can also ensure that your vehicle is in compliance with smog check regulations. This may involve performing a pre-smog check inspection to identify any potential issues and making the necessary repairs to ensure that your vehicle passes the test.

8. Key Considerations for Passing a Smog Check

Passing a smog check requires more than just resetting the OBD2 monitors. Here are some key considerations to keep in mind:

8.1. Regular Maintenance

Regular maintenance is essential for keeping your vehicle in good condition and preventing emission-related problems. This includes changing the oil, replacing the air filter, and performing other routine maintenance tasks.

8.2. Addressing Check Engine Light

If the check engine light is on, address the underlying issue as soon as possible. Ignoring the check engine light can lead to more serious problems and make it more difficult to pass a smog check.

8.3. Performing a Pre-Smog Check Inspection

Consider performing a pre-smog check inspection before taking your vehicle in for the official test. This will give you an opportunity to identify any potential issues and make the necessary repairs.

8.4. Understanding Smog Check Regulations

Familiarize yourself with the smog check regulations in your state. This will help you understand the requirements for passing the test and avoid any surprises.

9. Resources and Tools

Several resources and tools can help you with OBD2 monitor resetting and smog check preparation:

9.1. OBD2 Scan Tools

An OBD2 scan tool is an essential tool for diagnosing and resetting OBD2 monitors. There are many different scan tools available, ranging from basic models to more advanced ones with additional features.

9.2. Repair Manuals

A repair manual provides detailed information on your vehicle’s systems and components, including the OBD2 system. This can be helpful for understanding the drive cycle procedure and troubleshooting any issues.

9.3. Online Forums and Communities

Online forums and communities can be a valuable resource for getting advice and support from other vehicle owners and mechanics.

9.4. MERCEDES-DIAGNOSTIC-TOOL.EDU.VN

MERCEDES-DIAGNOSTIC-TOOL.EDU.VN offers a wealth of information on Mercedes-Benz diagnostics, repair, and maintenance. Their resources can help you understand the intricacies of your Mercedes-Benz OBD2 system and ensure it’s running optimally.

10. Disclaimer

The information provided in this article is intended for general guidance only. Always consult your vehicle’s owner’s manual and a qualified mechanic for specific instructions and recommendations.

FAQ: Resetting OBD2 Monitors for Smog Check

Here are some frequently asked questions about resetting OBD2 monitors for smog checks:

10.1. What happens if I just reset the monitors and go for a smog check?

Your vehicle will likely fail the smog check. Smog check technicians are trained to identify vehicles that have recently had their OBD2 systems reset. If the monitors haven’t had enough time to run and complete, it will raise a red flag, and your vehicle will likely fail the test.

10.2. How long does it take for OBD2 monitors to reset?

The time it takes for OBD2 monitors to reset varies depending on the vehicle make and model, the driving conditions, and the specific monitor. Some monitors may reset within a few minutes of driving, while others may take several days or even weeks.

10.3. Can I use a generic OBD2 scan tool to reset the monitors?

Yes, you can use a generic OBD2 scan tool to reset the monitors. However, some advanced functions may require a more specialized scan tool.

10.4. Is it legal to reset the OBD2 monitors to pass a smog check?

It is not illegal to reset the OBD2 monitors, but it is illegal to intentionally tamper with or disable emission control systems. If you reset the monitors without addressing the underlying cause of the problem, you may be in violation of smog check regulations.

10.5. What is a permanent diagnostic trouble code (PDTC)?

A permanent diagnostic trouble code (PDTC) is a type of diagnostic trouble code that cannot be erased by clearing the DTCs with a scan tool or by disconnecting the vehicle’s battery. PDTCs can only be cleared by the OBD II system once it has verified that the previously identified defects are no longer present.

10.6. How do I clear a PDTC?

PDTCs cannot be cleared manually. They will only clear after the OBD II system has verified that the underlying problem has been resolved and the monitor associated with the code has completed successfully.

10.7. What if my car fails the smog check due to a PDTC?

If your car fails the smog check due to a PDTC, you will need to have the underlying problem diagnosed and repaired. Once the repair has been completed, the PDTC will clear automatically after the monitor has run and completed successfully.

10.8. Are there any vehicles that are exempt from the PDTC check?

Yes, some vehicles are exempt from the PDTC check. In California, vehicles from model years 1996-2009 are exempt from the PDTC check. Additionally, some 2010 vehicles do not support PDTCs and will not be subject to the check.

10.9. What is the Modified Software Standard?

The Modified Software Standard is a check that was added to the BAR-OIS Smog Check inspection on July 19, 2021. Vehicles identified with illegally modified software will fail the Smog Check inspection.

10.10. What should I do if my car fails the smog check due to modified software?

If your car fails the smog check due to modified software, you will need to restore the vehicle to an OEM-approved or CARB-approved software configuration. Once the vehicle is restored, it may be retested at any licensed Smog Check station, unless otherwise directed by BAR to a Referee or STAR station.

Conclusion

Resetting OBD2 monitors can be a useful tool for troubleshooting emission-related problems and preparing for a smog check. However, it’s important to understand the process and potential issues involved. By following the tips and recommendations in this article, you can increase your chances of passing the smog check and keeping your vehicle running smoothly.

Do you have more questions about OBD2 diagnostics, unlocking hidden features, or maintaining your Mercedes-Benz? Contact MERCEDES-DIAGNOSTIC-TOOL.EDU.VN today for expert advice and solutions.

Contact Us:

• Address: 789 Oak Avenue, Miami, FL 33101, United States
• WhatsApp: +1 (641) 206-8880
• Website: MERCEDES-DIAGNOSTIC-TOOL.EDU.VN

Let us help you keep your Mercedes-Benz in top condition.