Passing a smog test can be stressful, especially if your car recently had maintenance that reset the onboard diagnostics. A key factor in passing emissions tests, particularly in California, is completing the necessary Obdii Drive Cycles. This process allows the vehicle’s computer to run checks on various emission control systems. This article provides a detailed guide on understanding and performing OBDII drive cycles to help your car pass its smog test.
Understanding OBDII Drive Cycles
OBDII (On-Board Diagnostics II) is a standardized system that monitors your car’s emissions components. After a battery disconnect or code clearing, the system needs to re-learn and verify the functionality of these components. This re-learning process is achieved through specific driving patterns known as OBDII drive cycles. These cycles activate various monitors, such as the catalyst, EVAP system, oxygen sensors, and EGR, allowing the system to assess their performance.
In California, failing a smog test can be attributed to incomplete monitors. The state allows a limited number of incomplete monitors (typically two), meaning if more than two system checks haven’t run, your vehicle will fail. That’s where understanding and performing OBDII drive cycles comes into play.
Performing an OBDII Drive Cycle
While general driving can sometimes complete these cycles, following a specific procedure ensures all necessary monitors run. This process involves a series of steps with varying speeds and conditions:
Pre-Conditions:
- Cold Start: Begin with a cold engine (coolant temperature below 122°F) and ensure the coolant and air temperature sensors are within 11°F of each other. Avoid turning the ignition key on before the cold start as this can prevent the oxygen sensor heater diagnostic from running.
- Fuel Level: Maintain a fuel tank level between ¼ and ¾ full.
Drive Cycle Steps:
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Idle: Immediately after starting the engine, idle in Drive (or Neutral for manual transmissions) for two and a half minutes with the A/C and rear defrost ON. This step checks the oxygen sensor heaters, air pump, and EVAP purge system.
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Acceleration: Turn off the A/C and rear defrost. Accelerate to 55 mph at half throttle. This tests the ignition system, fuel trim, and canister purge.
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Steady State Cruise: Maintain a constant speed of 55 mph for three minutes. This monitors the EGR system, air pump, oxygen sensors, and canister purge.
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Deceleration: Coast down to 20 mph without using the brakes or clutch. This step checks EGR and purge functions.
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Acceleration: Accelerate again to 55-60 mph at ¾ throttle. This further tests the ignition, fuel trim, and purge system.
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Steady State Cruise: Maintain a constant speed of 55-60 mph 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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Deceleration: Coast down to a complete stop without braking. This performs a final check of the EGR and canister purge system.
Verifying Completion and Passing the Smog Test
After completing the drive cycle, you can use an OBDII scanner (like the Autel AL619) to check the monitor status. Confirm that the previously incomplete monitors now show as “OK” or “Complete.” With the necessary monitors completed, your car should now be ready to pass the smog test.
Conclusion
Completing OBDII drive cycles is crucial for passing smog tests, especially after maintenance that resets the vehicle’s diagnostic system. Following the outlined steps and verifying completion with an OBDII scanner will significantly improve your chances of a successful smog test. Remember, driving behavior significantly impacts the completion of these cycles. While “just driving” might eventually complete them, following this specific procedure ensures all systems are thoroughly checked and ready for inspection.
