An Obdii Fail during a smog test can be frustrating, especially after a battery disconnect. California’s strict emissions regulations require all monitors to be ready, and a recent battery service can disrupt this process. This article outlines a specific driving cycle to help reset your car’s OBDII system and pass the smog test, based on personal experience with a ’97 Porsche and an Autel AL619 OBDII scanner.
Understanding the OBDII System and Smog Checks
When a car’s battery is disconnected, the On-Board Diagnostics II (OBDII) system loses its stored data, including information from various emissions monitors. These monitors track the performance of critical components like the catalytic converter, EVAP system, oxygen sensors, and EGR system. In California, only two incomplete monitors are allowed for a passing smog check. If more than two are not ready, the result is an “OBDII fail.”
A common scenario is a vehicle passing the emissions test but failing the OBDII systems readiness check. This often happens after a battery replacement or service, as disconnecting the battery clears the learned data in the OBDII system. The solution lies in completing a specific driving cycle to allow the system to re-evaluate these components.
The OBDII Drive Cycle: A Step-by-Step Guide
The following drive cycle is designed to activate the OBDII monitors. Ensure your fuel tank is between ¼ and ¾ full and begin with a cold start (coolant temperature below 122°F with coolant and air temperature sensor readings within 11°F of each other). Crucially, avoid turning the ignition on before the cold start to allow the oxygen sensor heater diagnostic to run.
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Idle: Immediately after starting the engine, idle in Drive (or Neutral for manual transmissions) for 2.5 minutes with the A/C and rear defrost ON. This checks the oxygen sensor heaters, air pump, and EVAP purge.
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Acceleration #1: Turn off the A/C and rear defrost. Accelerate to 55 mph at half throttle. This tests ignition misfire, fuel trim, and canister purge.
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Steady State #1: Maintain a constant speed of 55 mph for three minutes. This monitors the EGR, air pump, O2 sensors, and canister purge.
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Deceleration #1: Coast down to 20 mph without braking or using the clutch. This checks EGR and purge functions.
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Acceleration #2: Accelerate to 55-60 mph at ¾ throttle. This tests misfire, fuel trim, and purge again.
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Steady State #2: Maintain a constant speed of 55-60 mph for five minutes. This is a crucial step that monitors catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors, and purge functions.
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Deceleration #2: Coast down to a complete stop without braking. This performs a final check of the EGR and canister purge.
Confirmation and Passing the Smog Test
After completing the drive cycle, connect an OBDII scanner (like the Autel AL619) to check the monitor status. In my experience, completing steps 1-4 brought the O2 sensor and EGR monitors to “Ready.” After a subsequent smog test, the car passed. This specific drive cycle, unlike regular driving, effectively exercises the OBDII system, ensuring all monitors complete their diagnostic checks.
Conclusion: Overcoming an OBDII Fail
An “OBDII fail” after a battery disconnect doesn’t necessarily indicate a problem with your vehicle. It often just requires completing a specific driving cycle to allow the OBDII system to reset and monitor the emissions components. By following the outlined steps and using an OBDII scanner to verify monitor readiness, you can increase your chances of passing the California smog test.
