Resetting your vehicle’s OBD2 system, particularly the monitors, often requires performing a specific driving cycle. For General Motors (GM) vehicles, this process is meticulously outlined to ensure all diagnostic monitors are effectively evaluated. This guide provides a step-by-step approach to executing a GM OBDII driving cycle, crucial for clearing codes and confirming repair effectiveness.
Performing a GM OBDII Driving Cycle: A Step-by-Step Guide
The GM driving cycle is designed to run all OBDII diagnostic monitors. Completing this cycle can help reset your vehicle’s computer after repairs, allowing you to verify that the issues have been resolved and that your vehicle is ready for emissions testing. Follow these steps carefully to perform the cycle correctly:
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Cold Start Procedure: Begin with a cold start. For a valid cold start, the engine coolant temperature must be below 122°F (50°C) and within 11°F (6°C) of the ambient air temperature when you start the engine. Ensure the key is not in the ignition prior to the cold start, as this could interfere with the heated oxygen sensor diagnostic.
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Idle Phase: Once started, let the engine idle for two and a half minutes. During this idle period, turn on the air conditioner to maximum and engage the rear defroster. Increasing the electrical load on the engine during idle helps in testing several systems including the O2 sensor heater, Passive Air system, Purge “No Flow” diagnostics, and Misfire detection. If closed loop operation is achieved, this step also tests Fuel Trim.
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Acceleration: Turn off the air conditioner and the rear defroster, along with any other electrical loads. Accelerate smoothly using about half throttle until your vehicle reaches a speed of 55mph (88km/hr). This acceleration phase is essential for running diagnostics on Misfire, Fuel Trim, and Purge Flow systems.
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Maintain Steady Speed: Hold a constant speed of 55mph (88km/hr) for a duration of three minutes. Maintaining a steady speed allows the OBDII system to evaluate the O2 sensor response, Air Intrusive system, EGR (Exhaust Gas Recirculation), Purge system, Misfire detection, and Fuel Trim.
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Deceleration Phase: Release the accelerator pedal completely and allow the vehicle to coast down to 20 mph (32km/hr). It’s crucial during this deceleration phase not to shift gears or apply the brakes or clutch. This gradual coasting is important for testing the EGR system, Purge system, and Fuel Trim diagnostics.
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Second Acceleration: Accelerate again, this time using about 3/4 throttle, until you reach a speed between 55-60mph (88-96 km/hr). This step repeats the diagnostics performed in step 3, again focusing on Misfire, Fuel Trim, and Purge Flow.
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Sustained Steady Speed (Extended): Maintain a steady speed of 55mph (88km/hr) for a longer period of five minutes. In this extended steady speed phase, the system performs all the diagnostics from step 4, but with the addition of the crucial catalyst monitor diagnostics. If the catalytic converter is nearing the end of its life or if the battery has been recently disconnected, it might require up to five complete driving cycles to accurately assess the catalyst’s condition.
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Final Deceleration: Similar to step 5, release the accelerator pedal and coast down to 20 mph (32km/hr) without using the brakes, clutch, or shifting gears. This final deceleration again checks the EGR, Purge, and Fuel Trim systems.
By diligently following these steps, you can effectively perform a GM OBDII driving cycle. This process is instrumental in resetting your OBD2 monitors and ensuring your vehicle is correctly evaluated for emissions and overall system health. Remember to always prioritize safety and adhere to local traffic laws when performing a driving cycle.
