Are you facing a frustrating smog check failure due to your vehicle’s OBDII system? It’s a common issue, especially after battery replacements or when diagnostic trouble codes have been cleared. While your car might seem to run perfectly, the On-Board Diagnostic System (OBDII) might not be ready for inspection, leading to an unexpected fail. This article, brought to you by the automotive experts at autelfrance.com, will guide you through a specific OBDII drive cycle to help you complete those readiness monitors and pass your smog test.
The original poster experienced this exact problem. After a routine service involving battery disconnection, their 1997 vehicle failed the California smog check, not because of emissions levels, but due to incomplete OBDII monitors. Using an Autel AL619 OBDII scanner, they identified the issue: several monitors were in an “Incomplete” state. In California, and many other regions, vehicles are allowed only a limited number of incomplete monitors to pass the smog test. The solution isn’t just “driving it around”; it requires a specific driving pattern to activate and complete the necessary system checks.
Understanding OBDII Monitors and Readiness
OBDII systems utilize monitors to continuously evaluate the performance of your vehicle’s emission control systems. These monitors, such as the Catalyst Monitor, EVAP System Monitor, O2 Sensor Monitor, and EGR Monitor, run diagnostic tests under specific driving conditions. When these tests are not completed, the monitors are flagged as “Incomplete” or “Not Ready”. This often happens after events like battery disconnection because the system’s memory is cleared, and the monitors need to run their checks again.
Simply driving your car under normal conditions may not trigger all the necessary tests to complete these monitors. This is where the OBDII drive cycle comes in. A drive cycle is a predetermined series of driving maneuvers designed to create the specific conditions required for each monitor to run and complete its diagnostic test.
The OBDII Drive Cycle: Your Step-by-Step Guide to Smog Check Success
The following drive cycle is designed to exercise the necessary OBDII monitors. It begins with a cold start, crucial for initiating certain diagnostic routines. Ensure your engine coolant temperature is below 122 degrees Fahrenheit (50 degrees Celsius) and that the coolant and air temperature sensors are within 11 degrees Fahrenheit (6 degrees Celsius) of each other. Also, your fuel tank should be between 1/4 and 3/4 full. Important: Do not turn the ignition key to the “ON” position before the cold start, as this might prevent the heated oxygen sensor diagnostic from running.
Here’s the step-by-step OBDII drive cycle:
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Cold Start and Idle: Start the engine and immediately idle for two and a half minutes in drive (or neutral for manual transmissions as noted by the original poster). Turn on the air conditioning and rear defrost to increase engine load. This step checks the oxygen sensor heater circuits, air pump, and EVAP purge system.
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Acceleration Phase: Turn off the A/C and rear defrost. Accelerate to 55 mph (88 km/h) at half throttle. This stage assesses for ignition misfire, fuel trim, and canister purge.
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Steady Speed Driving (1): Maintain a steady speed of 55 mph (88 km/h) for three minutes. This allows the OBDII system to monitor the EGR, air pump, O2 sensors, and canister purge systems.
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Deceleration Phase (1): Coast down to 20 mph (32 km/h) without braking or depressing the clutch. This checks the EGR and purge functions during deceleration.
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Acceleration Phase (2): Accelerate again to 55 to 60 mph (88-96 km/h) at ¾ throttle. This re-evaluates misfire, fuel trim, and purge systems under a different load condition.
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Steady Speed Driving (2): Hold a steady speed of 55 to 60 mph (88-96 km/h) for five minutes. This extended steady speed driving is crucial for monitoring catalytic converter efficiency, misfire, EGR, fuel trim, oxygen sensors, and purge functions.
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Deceleration Phase (2): Decelerate and come to a complete stop without braking. This final coast-down allows the OBDII system to perform a last check of the EGR and canister purge systems.
After completing steps 1 through 4, the original poster checked their vehicle with an OBDII scanner and found that the O2 Sensor and EGR monitors were ready. They then took their vehicle for a smog check and successfully passed.
Tips for a Successful OBDII Drive Cycle
- Cold Start is Key: Ensure a proper cold start as described is critical for initiating the drive cycle correctly.
- Follow Speed and Throttle Instructions: Adhere to the specified speeds and throttle inputs as closely as possible for each step.
- Use an OBDII Scanner: Consider using an OBDII scanner like the Autel AL619 or similar to monitor the readiness status of your monitors before going for a smog check. This can save you time and prevent unnecessary retests.
- Safety First: Perform the drive cycle in a safe location where you can safely accelerate and decelerate without posing a risk to yourself or others.
- Patience is a Virtue: It may take more than one drive cycle to complete all monitors, especially if your vehicle has underlying issues.
By following this OBDII drive cycle guide, you can proactively address incomplete monitors and increase your chances of passing your next smog check. Remember to consult your vehicle’s repair manual for any specific drive cycle recommendations from the manufacturer, although as the original poster noted, these are not always easy to find even in detailed shop manuals. Good luck, and drive safely!
