If you’ve encountered a “Smog Check Obdii Fail” result, especially after battery replacement or service, you’re not alone. Many car owners face this issue when their vehicle fails the OBDII systems test despite passing the emissions test itself. This often stems from incomplete OBDII monitors, a common problem after disconnecting the car battery or clearing diagnostic trouble codes. Let’s delve into why this happens and, more importantly, how to resolve it using a specific OBDII drive cycle.
Understanding OBDII Monitor Readiness
Modern vehicles are equipped with sophisticated On-Board Diagnostic (OBDII) systems that continuously monitor various engine and emission control components. These monitors, such as those for the catalyst, EVAP system, O2 sensors, and EGR, ensure your car is running efficiently and cleanly. When these monitors are in a “ready” or “complete” state, it signifies they have been tested and are functioning correctly.
However, disconnecting your car battery or clearing trouble codes can reset these monitors to an “incomplete” state. This is because the system needs to re-run its diagnostic checks to confirm everything is still working as expected. In many states, including California, vehicles are allowed to have a limited number of incomplete monitors and still pass a smog check. However, exceeding this limit will result in a “smog check OBDII fail”.
Why OBDII Monitors Become Incomplete
The primary reason for incomplete monitors after a battery disconnect or code clearing is the erasure of the system’s memory. The OBDII system relies on specific driving patterns to execute its diagnostic tests and set the monitors to “complete”. Simply “driving it around” under normal conditions may not be sufficient to trigger all the necessary tests. This is where understanding and performing an OBDII drive cycle becomes crucial.
The OBDII Drive Cycle Solution
An OBDII drive cycle is a predefined series of driving conditions designed to force the OBDII system to run all its diagnostic monitors. By following this cycle, you can ensure that the incomplete monitors are tested and reset to a “ready” status, increasing your chances of passing your smog check. Here’s a step-by-step guide based on a proven method to address a “smog check OBDII fail” situation:
Step-by-Step Drive Cycle Instructions
Important Prerequisites:
- Cold Start: Begin with a cold engine. The coolant temperature should be below 122°F (50°C), and the ambient air temperature and coolant temperature should be within 11°F (6°C) of each other. Ensure the ignition key is not turned on before the cold start, as this might prevent the heated oxygen sensor diagnostic from running.
- Fuel Level: Your fuel tank should be between 1/4 and 3/4 full.
Drive Cycle Steps:
- Initial Idle: Start the engine and let it idle in drive (or neutral for manual transmissions) for two and a half minutes. Turn on the A/C and rear defroster during this step. This tests the oxygen sensor heater circuits, air pump, and EVAP purge system.
- Acceleration Phase: Turn off the A/C and rear defroster. Accelerate smoothly to 55 mph (88 km/h) at half throttle. This stage checks for ignition misfire, fuel trim, and canister purge.
- Steady Speed Cruising (1): Maintain a constant speed of 55 mph (88 km/h) for three minutes. This monitors the EGR, air pump, O2 sensors, and canister purge systems.
- Deceleration Phase (1): Coast down to 20 mph (32 km/h) without applying the brakes or depressing the clutch. This checks EGR and purge functions.
- Acceleration Phase (2): Accelerate again to 55 to 60 mph (88-96 km/h) at ¾ throttle. This re-examines misfire, fuel trim, and purge.
- Steady Speed Cruising (2): Hold a steady speed of 55 to 60 mph (88-96 km/h) for five minutes. This crucial step monitors catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors, and purge functions.
- Deceleration Phase (2): Coast down to a complete stop without using the brakes. This performs a final check of the EGR and canister purge.
Verifying Monitor Readiness
After completing the OBDII drive cycle, you can use an OBDII scanner, like the Autel AL619, to check the status of your monitors. Ideally, you should see most, if not all, monitors in a “ready” or “OK” state. In California, you are typically allowed up to two monitors to be “not ready” and still pass the smog check.
If your scanner indicates that the necessary monitors are now ready, you should be able to pass your smog check. It’s always a good idea to re-scan your vehicle at a smog check station to confirm before the official test.
Conclusion
Facing a “smog check OBDII fail” due to incomplete monitors can be frustrating, but it’s often easily resolved with the correct OBDII drive cycle. By following these steps, you can effectively reset your monitors and get your vehicle ready for a successful smog inspection. Remember, “just driving it” might not be enough; a specific drive cycle is often necessary to ensure all OBDII systems are properly tested and ready. Have you had success with this drive cycle method or have other tips for passing a smog check after an OBDII fail? Share your experiences in the comments below!
