Facing an OBDII smog test failure can be frustrating, especially after routine car maintenance like a battery replacement. Many car owners in areas with strict emissions regulations, like California, find themselves in this situation. If your car has recently had its battery disconnected or Diagnostic Trouble Codes (DTCs) cleared, you might encounter a smog check failure not due to excessive emissions, but because your vehicle’s On-Board Diagnostic System (OBDII) monitors are not ready. This guide provides a detailed, step-by-step OBDII drive cycle to help you complete these monitors and pass your smog test.
Understanding OBDII Monitor Readiness for Smog Tests
Modern vehicles utilize OBDII systems to continuously monitor engine and emission control components. These systems employ “monitors” to ensure everything is functioning correctly. After events like battery disconnection or code clearing, these monitors often reset to an “incomplete” or “not ready” status.
For a smog test to pass, especially in California and similar regions, your vehicle needs to have a certain number of monitors in a “ready” or “complete” state. Regulations typically allow for only one or two monitors to be incomplete, depending on the vehicle’s model year. If too many monitors are incomplete, your vehicle will fail the OBDII portion of the smog test, even if the emissions levels are within acceptable limits. Common incomplete monitors that can cause smog test failure include:
- Catalyst Monitor: Checks the efficiency of the catalytic converter.
- EVAP System Monitor: Evaluates the evaporative emissions control system for leaks.
- O2 Sensor Monitor: Tests the functionality of the oxygen sensors.
- EGR System Monitor: Monitors the exhaust gas recirculation system.
The key to passing an OBDII smog test when monitors are incomplete is to perform an OBDII drive cycle. This is a specific driving pattern designed to allow your vehicle to run through its diagnostic checks and set the monitors to a “ready” state. Simply “driving around” might not be enough, as the drive cycle requires specific conditions and maneuvers.
The OBDII Drive Cycle: Your Step-by-Step Guide to Smog Test Success
The following OBDII drive cycle is a general guideline and may need slight adjustments based on your vehicle’s make and model. Always consult your vehicle’s repair manual for the most accurate drive cycle information if available. However, this procedure is a great starting point for most vehicles.
Preparation Before Starting the OBDII Drive Cycle
To ensure the drive cycle executes correctly, consider these preparation steps:
- Cold Start: Begin the drive cycle from a cold start. This means the engine coolant temperature should be below 122 degrees Fahrenheit (50 degrees Celsius) and the ambient air temperature and coolant temperature should be within 11 degrees Fahrenheit (6 degrees Celsius) of each other. It’s best to let the car sit overnight before starting the drive cycle. Crucially, do not turn the ignition key to the “ON” position before the cold start. This can interfere with the heated oxygen sensor diagnostic.
- Fuel Level: Ensure your fuel tank is between 1/4 and 3/4 full.
Step-by-Step OBDII Drive Cycle Instructions
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Idle in Drive (or Neutral for Manual) with Accessories ON: Start the engine and immediately idle for two and a half minutes. Engage “Drive” for automatic transmissions or “Neutral” for manual transmissions. Turn on the air conditioning (A/C) and rear defroster. This step checks the oxygen sensor heater circuits, air pump, and EVAP purge system.
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Accelerate to 55 mph at Half Throttle: Turn off the A/C and rear defroster. Safely accelerate to 55 miles per hour using approximately half throttle. This initiates checks for ignition misfire, fuel trim, and canister purge.
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Maintain Steady Speed at 55 mph for Three Minutes: Hold a constant speed of 55 mph for three minutes. During this phase, the OBDII system monitors the EGR, air pump, O2 sensors, and canister purge systems.
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Decelerate to 20 mph Without Braking: Safely release the accelerator and coast down to 20 mph without applying the brakes or depressing the clutch. This step assesses EGR and purge functions.
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Accelerate to 55-60 mph at ¾ Throttle: Accelerate again to 55 to 60 mph, this time using about ¾ throttle. This re-checks for misfire, fuel trim, and purge system operation.
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Maintain Steady Speed at 55-60 mph for Five Minutes: Maintain a steady speed between 55 and 60 mph for five minutes. This is a crucial step for monitoring catalytic converter efficiency, misfire detection, EGR function, fuel trim, oxygen sensor performance, and purge functions.
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Decelerate to a Stop Without Braking: Finally, decelerate and come to a complete stop without using the brakes. This allows the OBDII system to perform a final check of the EGR and canister purge systems.
Verifying Monitor Completion After the Drive Cycle
After completing the OBDII drive cycle, you can check if the monitors have reset to “ready” using an OBDII scanner. Connect a scanner to your vehicle’s OBDII port. Many affordable scanners, like the Autel AL619, can read monitor status. Check the “Monitor Status” or “I/M Readiness” section of the scanner. You should see more monitors marked as “OK” or “Complete” compared to before the drive cycle.
If you have completed the drive cycle correctly, and your vehicle is functioning properly, you should see that the necessary monitors are now in a “ready” state. You can then take your vehicle for a smog test with confidence.
Conclusion: Drive Cycles for Smog Test Success
Passing an OBDII smog test after a battery disconnect or code clearing doesn’t have to be a mystery. By understanding the importance of OBDII monitor readiness and performing a specific drive cycle, you can effectively prepare your vehicle for the test. This step-by-step guide provides a practical approach to completing those monitors, increasing your chances of a successful smog test outcome and getting back on the road legally. Instead of simply “driving it,” take control and utilize the OBDII drive cycle to ensure your car is ready for its emissions inspection.
