Facing a smog test failure due to OBDII issues can be frustrating, especially after routine car maintenance like battery replacement. Many car owners encounter this problem, where the vehicle passes the emissions test itself but fails the OBD II systems check. This often occurs because disconnecting the battery or clearing diagnostic codes resets the OBDII system’s monitors, which need to be in a “ready” state to pass inspection. This article provides a clear explanation and a practical drive cycle solution to address the “Obdii Fail Smog” situation, ensuring your vehicle is ready for its next smog test.
The issue arises when your car’s OBDII system reports “incomplete” monitors. These monitors are diagnostic checks that the car’s computer performs on various emission control systems, such as the catalyst, EVAP system, oxygen sensors, and EGR. When these monitors are not ready, it indicates that the system hasn’t completed its self-testing cycle since the last reset. California, like many other states, has specific regulations regarding the number of incomplete monitors allowed to pass a smog test. Typically, vehicles are allowed only one or two incomplete monitors to still pass the OBDII portion of the smog test. If you have more than the allowed limit, you will fail, even if your car is actually running cleanly.
To resolve this “OBDII fail smog” scenario, you need to perform an OBDII drive cycle. This drive cycle is a specific series of driving conditions designed to exercise your car’s emission control systems and allow the OBDII monitors to complete their self-tests and switch to a “ready” or “complete” status. The following drive cycle is a commonly recommended procedure to reset these monitors. It’s crucial to start with a “cold start,” meaning the engine coolant temperature is below 122 degrees Fahrenheit (50 degrees Celsius) and close to the ambient air temperature. Also, ensure your fuel tank is between 1/4 and 3/4 full.
Here are the steps for the OBDII drive cycle:
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Cold Start and Idle: Start the engine and let it idle in drive (or neutral for manual transmissions) 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 and Moderate Speed: Turn off the air conditioning and rear defroster. Accelerate smoothly to 55 mph (88 km/h) at half throttle. This action initiates checks for ignition misfire, fuel trim, and canister purge.
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Maintain Steady Speed: Hold a steady speed of 55 mph (88 km/h) for three minutes. During this phase, the OBDII system monitors the EGR, air pump, oxygen sensors, and canister purge system.
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Deceleration: Coast down to 20 mph (32 km/h) without using the brakes or clutch. This step allows the OBDII to check EGR and purge functions during deceleration.
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Re-acceleration: Accelerate again to 55 to 60 mph (88-96 km/h) at ¾ throttle. This re-checks misfire, fuel trim, and purge systems under a slightly higher load.
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Sustained High-Speed Cruise: Maintain a steady speed of 55 to 60 mph (88-96 km/h) for five minutes. This extended cruise monitors catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors, and purge functions.
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Final Deceleration and Stop: Coast down to a complete stop without braking. This final deceleration allows a last check of the EGR and canister purge systems.
After performing a few steps of this drive cycle, you can use an OBDII scanner to check the status of your monitors. A tool like the Autel AL619 (mentioned in the original post) or similar OBDII code readers can be very helpful to monitor the readiness status of each system. You may not need to complete the entire drive cycle for all monitors to become ready. Once you see that the necessary monitors (Catalyst, EVAP, O2 Sensor, EGR in the original example) are showing as “OK” or “Complete,” you can take your vehicle for a smog test.
In conclusion, if you’re facing an “OBDII fail smog” situation after a battery disconnect or code reset, performing an OBDII drive cycle is often the solution. By following these steps, you can ensure your vehicle’s monitors are ready, allowing you to pass the smog test and get back on the road with confidence. Remember to consult your vehicle’s repair manual or a trusted mechanic for specific drive cycle recommendations for your car model if needed.
