Encountering a check engine light can be concerning, often signaling that your vehicle has stored OBD2 codes related to various system malfunctions. While some issues require professional attention, a specific procedure known as a driving cycle can help clear certain OBD2 codes, particularly after repairs have been made. This guide focuses on the General Motors (GM) OBDII driving cycle, a series of steps designed to run onboard diagnostic monitors and reset them, potentially clearing codes in the process.
This driving cycle is most effective after you’ve addressed the underlying issue causing the fault code. For instance, if you’ve replaced a faulty oxygen sensor, completing a GM driving cycle can help the system re-evaluate the sensor’s performance and clear the related code if the repair was successful. It’s important to understand that this procedure is not a guaranteed fix for all check engine light problems, but it’s a valuable step in verifying repairs and ensuring your vehicle’s emission systems are functioning correctly.
Here’s a step-by-step guide to performing a GM OBDII driving cycle:
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Cold Start: Begin with a cold engine. 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 vehicle. 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 with Load: Start the engine and let it idle for two and a half minutes. Turn on the air conditioner and the rear defroster to increase the electrical load. This step is crucial for testing components like the O2 heater circuit, passive air system, purge valve (for “no flow” conditions), and misfire detection. If the system achieves closed loop operation, it will also assess fuel trim.
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Moderate Acceleration: Turn off the air conditioner and all other electrical loads. Accelerate smoothly using about half throttle until you reach a speed of 55mph (88km/hr). This acceleration phase allows the system to further evaluate misfire, fuel trim, and purge flow diagnostics under changing engine load.
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Maintain Steady Speed: Hold a constant speed of 55mph (88km/hr) for three minutes. During this steady cruise, several critical emission monitors are activated, including O2 sensor response, air intrusion, EGR (Exhaust Gas Recirculation) system, purge control, misfire, and fuel trim diagnostics.
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Deceleration: Release the accelerator pedal completely and allow the vehicle to coast down to 20 mph (32km/hr) without shifting gears, braking, or depressing the clutch. This deceleration phase is important for assessing the EGR system, purge control, and fuel trim under different engine conditions.
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Accelerate Again: Apply approximately 3/4 throttle and accelerate back up to 55-60mph (88-96 km/hr). This step repeats the diagnostics performed during the initial acceleration phase, again focusing on misfire, fuel trim, and purge flow.
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Extended Steady Speed: Maintain a steady speed of 55mph (88km/hr) for a longer duration of five minutes. This extended period at a constant speed is essential for running the catalyst monitor diagnostics, in addition to the monitors tested in step 4. If the catalytic converter’s performance is marginal, 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: Once again, release the accelerator pedal and coast down to 20 mph (32km/hr) without using the brakes, clutch, or shifting gears. This final deceleration step again evaluates the EGR system, purge control, and fuel trim as the vehicle slows down.
Important Considerations:
- Safety First: Perform this driving cycle in a safe location where you can legally and safely reach and maintain the specified speeds. Avoid heavy traffic areas.
- Not a Universal Solution: This GM driving cycle is specifically designed for General Motors vehicles. Other manufacturers may have different driving cycle procedures.
- Underlying Issues: Clearing OBD2 codes with a driving cycle doesn’t fix the root problem. It only resets the monitors after a repair. If the check engine light returns, there’s likely still an issue that needs to be addressed.
- OBD2 Scanner: For confirmation that codes have been cleared or to force clear codes, consider using an OBD2 scanner. These tools can read and clear diagnostic trouble codes directly.
By following these steps, you can effectively perform a GM OBDII driving cycle, which is a valuable procedure for verifying emission system repairs and potentially clearing OBD2 codes on your GM vehicle. Remember to consult your vehicle’s repair manual or a qualified mechanic for specific diagnostic and repair guidance.
