Facing an OBDII smog check failure can be frustrating, especially after routine car maintenance like a battery replacement. You might have a clean check engine light and no fault codes, yet still fail the smog test. This often stems from “incomplete” OBDII monitors. If you’re in California or any region with stringent emissions testing, understanding and resolving this issue is crucial. As an auto repair expert at autelfrance.com, I’m here to guide you through understanding why this happens and, more importantly, how to fix it using an OBDII drive cycle.
Understanding OBDII Monitors and Smog Tests
Modern vehicles are equipped with sophisticated On-Board Diagnostic (OBDII) systems that continuously monitor various engine and emission control components. These checks are performed by “monitors,” which are essentially diagnostic routines. When your car undergoes a smog test, the testing station not only checks for fault codes but also verifies the status of these monitors.
After a car battery disconnection or clearing diagnostic trouble codes (DTCs), these monitors often reset to an “incomplete” or “not ready” state. This is because the OBDII system requires the car to be driven under specific conditions to run and complete these self-tests. California, and many other states, allow a limited number of incomplete monitors (typically one or two, depending on the vehicle year) to pass the smog test. Exceeding this limit, even without any apparent engine problems, will result in an Obdii Smog Failure.
The key monitors often involved in incomplete status issues include:
- Catalyst Monitor: Checks the efficiency of the catalytic converter in reducing harmful emissions.
- EVAP System Monitor (Evaporative System): Tests for leaks in the evaporative emission control system, which prevents fuel vapor release.
- O2 Sensor Monitor (Oxygen Sensor): Evaluates the performance of the oxygen sensors, crucial for fuel mixture control and emissions.
- EGR Monitor (Exhaust Gas Recirculation): Monitors the EGR system, which reduces NOx emissions by recirculating exhaust gas back into the engine intake.
The OBDII Drive Cycle: Your Solution to Smog Fail
The solution to clear incomplete monitors and pass your smog test is often simpler than you might think: performing an OBDII drive cycle. A drive cycle is a pre-programmed series of driving conditions designed to force the OBDII system to run and complete all emission monitors. Simply “driving it around” usually isn’t sufficient because normal driving patterns may not meet the specific criteria required to trigger each monitor.
The following OBDII drive cycle is a general guideline and might need slight adjustments based on your specific vehicle make and model. Always consult your vehicle’s repair manual or a trusted mechanic for the most accurate drive cycle for your car. However, this procedure is a great starting point and has proven effective for many vehicles:
Step-by-Step OBDII Drive Cycle Instructions
Important Pre-Conditions:
- Cold Start: Begin with a cold start. The coolant temperature should be below 122 degrees Fahrenheit (50 degrees Celsius) and the intake air temperature should be within 11 degrees Fahrenheit (6 degrees Celsius) of the coolant temperature. Ensure the ignition key is off prior to the cold start to allow the heated oxygen sensor diagnostic to run correctly.
- Fuel Level: Your fuel tank should be between 1/4 and 3/4 full.
Drive Cycle Steps:
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Idle Phase: Start the engine and let it idle in drive (or neutral for manual transmissions as noted in the original article, though ‘drive’ is generally recommended for most drive cycles) for two and a half minutes. Turn on the air conditioning and rear defroster to increase engine load. This step checks the oxygen sensor heater circuits, air pump, and EVAP purge system.
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Acceleration Phase: Turn off the air conditioning and rear defroster. Accelerate smoothly to 55 mph (88 km/h) at half throttle. This engages tests for ignition misfire, fuel trim, and canister purge.
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Steady Speed Phase (1): Maintain a steady speed of 55 mph (88 km/h) for three minutes. During this phase, the OBDII system monitors the EGR, air pump, O2 sensors, and canister purge systems.
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Deceleration Phase: Coast down to 20 mph (32 km/h) without braking or depressing the clutch. This action checks the EGR and purge functions.
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Acceleration Phase (2): Accelerate again to 55 to 60 mph (88-96 km/h) at ¾ throttle. This re-engages checks for misfire, fuel trim, and purge.
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Steady Speed Phase (2): Hold a steady speed of 55 to 60 mph (88-96 km/h) for five minutes. This is a crucial step for monitoring catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors, and purge functions.
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Final Deceleration Phase: Coast down to a complete stop without braking. This final deceleration allows the OBDII system to perform a last check of the EGR and canister purge systems.
Post-Drive Cycle Check:
After completing these steps, you can use an OBDII scanner, like the Autel AL619 mentioned in the original article (or any Autel OBDII scanner from autelfrance.com), to check the status of your monitors. Ideally, you should see that more monitors are now reading “OK” or “Complete.”
In the original author’s experience, after steps 1-4, their OBDII scanner showed the O2 Sensor and EGR monitors as “OK.” They then took their vehicle for a smog test and successfully passed.
Tips for a Successful OBDII Drive Cycle
- Safety First: Perform the drive cycle in a safe location where you can legally and safely perform the accelerations and decelerations. Avoid heavy traffic areas.
- Patience is Key: It might take more than one drive cycle to complete all monitors. Don’t be discouraged if they don’t all clear immediately.
- Use an OBDII Scanner: Investing in an OBDII scanner is highly recommended. It allows you to monitor the progress of your drive cycle in real-time and confirm when the necessary monitors are ready. Autel offers a range of scanners suitable for DIYers and professionals alike.
- Consult Your Vehicle Manual: While the provided drive cycle is general, your vehicle’s repair manual may have a specific drive cycle recommended by the manufacturer. Always prioritize manufacturer recommendations when available.
- Consider Professional Help: If you are still struggling to complete the monitors after several drive cycles, there might be an underlying issue with your vehicle. Consult a certified mechanic for a professional diagnosis.
Conclusion
An OBDII smog fail due to incomplete monitors after a battery disconnect or code clearing is a common issue, but it’s often easily resolvable. By understanding the OBDII system and performing a proper drive cycle, you can often get your vehicle ready to pass the smog test without expensive repairs. Remember to use an OBDII scanner to track your progress and ensure you’re performing the drive cycle effectively. With the right approach, you can confidently tackle that smog test and keep your vehicle compliant with emissions standards.
