Decoding Fail OBD2 Monitors: A Comprehensive Guide for Vehicle Owners and Technicians

Navigating the complexities of modern vehicle diagnostics can be daunting, especially when faced with the cryptic message of “Fail Obd2 Monitors.” As a content creator at techcarusa.com and a seasoned auto repair expert, I’m here to demystify this issue. This guide provides an in-depth look into OBD2 monitors, why they might fail, and how to address these failures effectively, ensuring your vehicle meets emission standards and runs optimally.

Understanding OBD2 Monitors and Smog Check Inspections

On-Board Diagnostics (OBD) systems are integral to modern vehicles, acting as the vehicle’s self-diagnostic and reporting system. Since the 1990s, OBD systems, particularly OBDII, have become standardized, offering a wealth of information about a vehicle’s health, especially concerning emission control. A crucial part of the OBD system is the suite of “readiness monitors.” These are self-tests that the vehicle’s computer (Powertrain Control Module or PCM) performs to ensure all emission control systems are functioning correctly.

When you take your vehicle for a smog check or emission test, the OBDII system is scanned. This scan checks for several things, including:

• Malfunction Indicator Lamp (MIL) status: Is the “Check Engine” light on or off?
• Diagnostic Trouble Codes (DTCs): Are there any stored fault codes indicating problems?
• Readiness Monitor Status: Have the required emission monitors run and completed their self-tests?
• Communication: Can the testing equipment communicate with the vehicle’s computer?
• Permanent Diagnostic Trouble Codes (PDTCs): Are there any fault codes that cannot be cleared by simply resetting the system?
• Modified Software: Has the vehicle’s software been illegally modified?

A “fail OBD2 monitors” result typically refers to the readiness monitors not being in a “ready” or “complete” state. This article will primarily focus on these monitors, but we will also touch upon other aspects of OBDII testing failures.

Readiness Monitors: The Gatekeepers of Emission Compliance

Readiness monitors are designed to confirm that your vehicle’s emission control systems are working as intended. Think of them as checkpoints in a system of tests. There are two main categories of monitors:

• Continuous Monitors: These run constantly while the engine is operating. They include:

  • Misfire Monitor: Detects engine misfires that can increase emissions.
  • Fuel System Monitor: Checks the fuel delivery and metering system.
  • Comprehensive Components Monitor (CCM): Oversees a wide range of sensors and actuators for proper function.

• Non-Continuous Monitors: These run under specific driving conditions. They include:

  • Catalyst Monitor: Verifies the efficiency of the catalytic converter.
  • Evaporative System Monitor: Checks for fuel vapor leaks in the EVAP system.
  • Oxygen Sensor Monitor: Tests the functionality of oxygen sensors.
  • Oxygen Sensor Heater Monitor: Checks the heaters for the oxygen sensors.
  • EGR System Monitor: (Exhaust Gas Recirculation) Verifies the EGR system operation.
  • Secondary Air System Monitor: Checks the secondary air injection system (if equipped).
  • Heated Catalyst Monitor: Tests heated catalytic converters (if equipped).
  • A/C System Refrigerant Monitor: (For R134a or newer systems) Checks for refrigerant leaks (some vehicles).
  • NMHC Catalyst Monitor: (Non-Methane Hydrocarbon) For advanced catalyst systems.
  • NOx Aftertreatment Monitor: (Nitrogen Oxides) For diesel and some gasoline vehicles with NOx reduction systems.
  • Particulate Filter Monitor: (Diesel Particulate Filter – DPF) For diesel vehicles.
  • Boost Pressure Control System Monitor: For turbocharged vehicles.
  • Exhaust Gas Sensor Monitor: Monitors sensors related to exhaust gas composition.

Alt text: Diagram illustrating the On-Board Diagnostic (OBD) system in a vehicle, highlighting the connection between sensors, engine control unit (ECU), and diagnostic tools.

Why Do OBD2 Monitors Fail to Set? Common Causes

When a readiness monitor is “not ready” or “incomplete,” it means the self-test has not run or has not completed successfully. This can be due to several reasons, not all of which indicate a problem with your vehicle. Here are common causes for fail OBD2 monitors:

1. Recent Battery Disconnection or DTC Reset: This is the most frequent reason. When the battery is disconnected or trouble codes are cleared using a scan tool, all readiness monitors are reset to an “incomplete” state. The vehicle needs to be driven through specific “drive cycles” to allow these monitors to run again.

2. Drive Cycle Not Completed: Each monitor has specific criteria that must be met for it to run. These criteria, known as a “drive cycle,” involve a combination of driving conditions like speed, engine load, temperature, and duration. If you haven’t driven your car in a way that meets these conditions since the last reset, monitors may remain incomplete. OEM drive cycles can be complex and are often found in service manuals.

3. Underlying Emission System Faults: If a genuine issue exists within an emission control system, the corresponding monitor may fail to complete because the system is not functioning correctly. For example, a faulty oxygen sensor can prevent the oxygen sensor monitor from setting.

4. Vehicle Design Anomalies: Some vehicles, due to their design or software, have known difficulties in setting certain monitors. The original article provides extensive tables detailing vehicles with such issues. In these cases, the OBD test system might be programmed to accommodate these known anomalies, or specific repair procedures are required.

5. Software Issues: Problems in the vehicle’s engine control software can also prevent monitors from running correctly. Software updates or reflashing the PCM might be necessary to resolve these issues, as highlighted in the original document for various BMW and Chrysler/Dodge/Jeep models.

6. Permanent Diagnostic Trouble Codes (PDTCs): The presence of PDTCs, even if the MIL is off, can sometimes affect readiness monitor status. PDTCs indicate a confirmed fault that the OBD system recognizes as still unresolved, even if it’s not currently triggering the “Check Engine” light.

7. Modified or Unapproved Software: Tampering with the vehicle’s software or using non-OEM approved software can lead to OBD system malfunctions, including readiness monitor issues and outright test failures due to software modification detection.

Pass/Fail Standards for OBD2 Monitor Readiness

The number of incomplete monitors allowed to pass an OBD test varies based on the vehicle’s model year and fuel type. Understanding these standards is crucial for both vehicle owners and technicians preparing for a smog check.

Model Year	Fuel Type	Number of Incomplete Monitors Allowed to Pass OBD Test¹
1996-1999	Gas²	Any one³
2000 and newer	Gas²	Evaporative system³
1998-2006	Diesel	Zero
2007 and newer	Diesel	Particulate Filter and Non-Methane HydroCarbon (NMHC)⁴

¹ Implemented May 4, 2015.
² “Gas” includes gasoline, propane, natural gas (CNG, LNG, LPG).
³ The misfire, fuel, and comprehensive component continuous monitors are ignored during an OBD test on “gas” vehicles.
⁴ Implemented July 5, 2023.

Key Takeaways from the Standards:

• Older Gasoline Vehicles (1996-1999): More lenient, allowing one incomplete monitor (excluding continuous monitors).
• Newer Gasoline Vehicles (2000 and newer): Stricter, only allowing the evaporative system monitor to be incomplete.
• Older Diesels (1998-2006): Very strict, requiring all monitors to be complete.
• Newer Diesels (2007 and newer): Allow two specific monitors (Particulate Filter and NMHC) to be incomplete.

It’s important to note that continuous monitors (Misfire, Fuel System, and Comprehensive Components) are generally ignored during OBD tests on gasoline vehicles. However, they must still function correctly for the vehicle to operate properly and for non-continuous monitors to set.

Vehicle-Specific OBD2 Monitor Issues and Solutions

The original document meticulously lists numerous vehicle makes and models known to have OBD test issues. These issues range from communication problems to specific monitors that are difficult to set. Here’s a summarized overview of common vehicle-specific problems and the recommended actions:

1. Communication Failures:

• Audi & VW (2000-2005 Passat, A4 & A6): No OBD communication due to a broken ground wire in the ABS module. Repairing the ground wire as per TSB resolves this.
• Ford (Various 2020-2025 models like Escape, Bronco Sport, Explorer, F-Series): Communication issues often linked to gateway module software. Software updates from dealerships are the solution. Some newer models (2025+ Explorer) use OBDonUDS, requiring updated diagnostic tools.
• BMW (2001-2003 Z3, Z8, M5, M3): Intermittent communication issues. Following OIS prompts to connect/disconnect the DAD with the key off can help. Software updates (recalls) also address this.
• Hyundai (2003 Tiburon): Communication problems due to pin 5 not being grounded in the DLC. Grounding pin 5 resolves this.
• GM (2000-2008 Various models): “No vehicle voltage” prompts may indicate a blown fuse or power issue to the OBD connector. Using auxiliary power cables for the DAD can bypass this.

2. Readiness Monitor Setting Difficulties:

• BMW (Various 1998-2000 models): Oxygen sensor monitor issues due to DME software errors. Reprogramming the DME with updated software (TSB #SI B12 16 16, Recall #00E-A01) is required.
• Chevrolet (2001 Silverado/Tahoe, 2002 Silverado HD Diesel): Secondary air system and EGR monitor issues. Diagnose and repair underlying system faults. For some models (Silverado HD diesel EGR), OIS is reprogrammed to allow the monitor to be incomplete.
• Chrysler/Dodge/Plymouth (1999-2000 Town & Country, Caravan, Voyager 3.0L): EGR monitor issues at high altitude. PCM reflash (TSB 18-03-00) is the solution.
• Ford (2003 Focus, 2011 Edge/Explorer/Lincoln MKX): General readiness monitor setting difficulties. PCM reflash (TSB #06-7-5 for Focus, Recall #13E04 for Edge/Explorer/MKX) and completing drive cycles are needed.
• Hyundai (2002 Sonata), Suzuki (2001 Aerio, 2004 Forenza), Toyota (2000-2006 All models), Nissan (2000-2006 All models): General monitor setting difficulties. Following specific drive cycles (OEM or OBD Clearinghouse) is crucial.
• Jeep (2010-2011 Wrangler 3.8L): Oxygen sensor monitor issues. PCM reflash (TSB 18-027-10, Recall L34/L35) and specific drive cycles are recommended.
• Kia (2009 Borrego 3.8L): EGR monitor issues. OIS is reprogrammed to allow the EGR/VVT monitor to be incomplete.
• Mazda (2000 MPV, 2002-2003 B3000): Catalyst monitor issues (MPV), readiness monitor issues on flex-fuel models (B3000). Diagnose and repair catalyst issues. OIS ignores monitors on certain flex-fuel models.
• Mitsubishi (2000-2001 Montero, etc.): General monitor setting difficulties. Following recommended drive cycles in the service manual is key.
• Volvo (2000 S40/V40): Comprehensive, Oxygen sensor, and Catalyst monitor failures due to defective ECM. Referee inspection is recommended if only these monitors are unset.

3. False MIL Illumination/Incorrect Readings:

• Dodge (2003 Ram 5.9L Diesel): MIL commanded on but no DTCs stored. Vehicle design anomaly. OIS ignores MIL command if no DTCs are present. Recall #K01 reflash for California certified vehicles.
• Mazda (2003-2005 Mazda 6): False MIL illumination and U-codes during OBDII test due to PCM sensitivity to tool connection/disconnection. Connect/disconnect DAD only with key off. Software updates (TSB #01-024/05, #01-002/05, Recall #1303F) may be needed.
• Mercedes-Benz (1996 C-Class, E-Class, S-Class, SL-Class): MIL illuminates when scan tool connected if OBDII connector is used for RPM source. Use conventional RPM measurement methods and connect OBDII only when prompted.

4. Permanent Diagnostic Trouble Code (PDTC) Issues:

• Chrysler/Dodge/Jeep/Ram (Various 2010-2019 models): PDTCs that won’t clear, often U0140 or related to specific systems. Reflash per relevant Recalls (V51, V50, TSBs like 08-005-21, 08-008-21, etc.) is required.
• Audi & VW (2010-2012 Q7, Touareg Diesel): Specific PDTCs (P0087, P0088, P053F, etc.) that won’t clear. BAR-OIS ignores these specific codes.
• Ford (2011-2014 Mustang): PDTC P0315 not self-clearing. Customer Satisfaction Program 17N01 is available at Ford dealerships.
• Freightliner/Mercedes (2014-2015 Sprinter Diesel), Jeep (2014-2017 Grand Cherokee Diesel), Ram (2014-2017 1500 Diesel), Mercedes (2010-2013 350 Bluetec): Various PDTCs (P202A, P203A, P208A, P20B9, P20BD, P20BE, P20FA, P1D30, P203D, P20B6, etc.) that are ignored by BAR-OIS. Test normally; these are known vehicle anomalies.

5. Vehicles Not OBDII Certified (Federal Diesel Vehicles over 8,500 GVWR – e.g., Chevy/GMC C/K Series, Ford F/E Series, Dodge Ram, Freightliner M Line/Sprinter, Isuzu NPR, Mitsubishi Fuso, Nissan UD): These vehicles may not fully comply with OBDII standards. OIS is programmed to handle this by ignoring incomplete continuous monitors or skipping OBD functional tests. Bulb checks may be used for OBD test results if communication fails.

6. Flex Fuel Vehicle Specifics (Chevy/GMC S-10/Sonoma, Chrysler Town & Country, Plymouth Voyager, Mazda B3000 E-85 models): Monitors may be disabled or desensitized when running on higher ethanol blends. OIS may ignore all or specific monitors on these vehicles.

It is crucial to consult the detailed tables in the original document for the complete list of affected vehicles, specific model years, engine types, and precise instructions for inspectors and technicians.

Troubleshooting Fail OBD2 Monitors: A Step-by-Step Approach

If your vehicle fails a smog check due to incomplete readiness monitors, or if you are proactively trying to ensure your vehicle is ready, follow these steps:

1. Verify the Reason for Incomplete Monitors: Has the battery been recently disconnected? Have DTCs been cleared? If so, a drive cycle is likely needed.

2. Perform a Drive Cycle: Consult your vehicle’s owner’s manual or a reliable source like the OBD Clearinghouse or a repair information service for the specific drive cycle for your vehicle. Drive cycles can be complex and must be performed precisely. Be aware of traffic laws and safety while performing drive cycles.

3. Check for Diagnostic Trouble Codes (DTCs): Use an OBDII scan tool to check for any stored DTCs. Address any fault codes first, as they may be preventing monitors from setting. Repair any identified issues.

4. Inspect for Vehicle-Specific Issues: Refer to resources like the original document provided or online databases for known OBD issues related to your vehicle’s make, model, and year. Check for relevant Technical Service Bulletins (TSBs), recalls, or customer satisfaction programs.

5. Allow Sufficient Driving Time: After performing a drive cycle or repairs, allow for several days of normal driving. Some monitors, especially evaporative system monitors, can take time and multiple drive cycles to complete.

6. Re-Scan for Readiness Status: Use an OBDII scan tool to re-check the readiness monitor status before going for another smog check. Ensure the necessary monitors are “ready.”

7. Consider Professional Diagnosis: If monitors still remain incomplete after drive cycles and troubleshooting, seek professional diagnosis from a qualified mechanic. They can use advanced diagnostic tools to pinpoint underlying issues preventing monitor completion.

8. Software Updates and Reflashes: For known software-related issues, as indicated in the vehicle-specific lists, consider getting the PCM reflashed or software updated at a dealership. This is often covered under warranty or recalls.

9. Check for Permanent Diagnostic Trouble Codes (PDTCs): Ensure there are no PDTCs present, as these can also cause test failures even if the MIL is off. PDTCs require the underlying issue to be resolved and the OBD system to self-clear them over time.

Conclusion: Ensuring OBD2 Monitor Readiness for Emission Compliance

“Fail OBD2 monitors” doesn’t always indicate a serious problem, but it does mean your vehicle is not ready for an emission test. Understanding the reasons behind incomplete monitors, the pass/fail standards, and vehicle-specific issues is crucial for both vehicle owners and technicians.

By following a systematic approach to troubleshooting, performing drive cycles correctly, addressing underlying faults, and utilizing resources like vehicle-specific issue lists and repair information, you can effectively resolve fail OBD2 monitor situations. Ensuring your OBD2 monitors are “ready” not only allows your vehicle to pass smog checks but also confirms that your emission control systems are functioning correctly, contributing to cleaner air and optimal vehicle performance.

For further in-depth information and vehicle-specific guidance, always consult your vehicle’s service manual, OEM technical resources, and reputable databases like the OBD Clearinghouse. Staying informed and proactive is the key to successfully navigating the world of OBD2 diagnostics and ensuring your vehicle is emission compliant.