A GM OBD2 driving cycle is a specific series of driving conditions that are required to allow your General Motors vehicle’s onboard computer to run self-tests, also known as monitors. Completing a driving cycle is often necessary after battery disconnection or repairs to ensure your vehicle is ready for an emissions test. This guide outlines the steps to properly perform a GM OBD2 driving cycle.
Step-by-Step GM OBD2 Driving Cycle
Follow these steps in order to complete a GM OBD2 driving cycle for your General Motors vehicle.
1. Cold Start
To initiate the driving cycle, a cold start is crucial. Ensure the engine coolant temperature is below 122°F (50°C). Ideally, it should be within 11°F (6°C) of the ambient air temperature at startup. Avoid turning the ignition key to the ‘on’ position prior to the cold start, as this can prevent the heated oxygen sensor diagnostic from running correctly.
2. Idle
Once started, let the engine idle for two and a half minutes. Turn on the air conditioner and the rear defroster to increase the electrical load on the engine. This step is important for testing several monitors including:
- O2 Heater: Checks the functionality of the oxygen sensor heaters.
- Passive Air: Tests the passive air injection system (if equipped).
- Purge “No Flow”: Diagnoses issues with the evaporative emission control system purge flow.
- Misfire: Detects engine misfires.
- Fuel Trim: Evaluates the engine’s fuel delivery system (if closed loop operation is achieved).
3. Accelerate
Turn off the air conditioner and rear defroster, and any other unnecessary electrical loads. Gently accelerate to 55mph (88km/hr) using about half throttle. During this acceleration phase, the following diagnostics are performed:
- Misfire: Continues misfire detection under load.
- Fuel Trim: Further evaluation of the fuel system.
- Purge Flow: Checks the evaporative emission system purge flow again.
4. Hold Steady Speed
Maintain a constant speed of 55mph (88km/hr) for three minutes. During this steady cruise, several important monitors are activated:
- O2 Response: Tests the response time of the oxygen sensors.
- Air Intrusive: Evaluates the air injection system.
- EGR (Exhaust Gas Recirculation): Checks the EGR system operation.
- Purge: Another check of the evaporative emission system purge function.
- Misfire: Continuous misfire monitoring.
- Fuel Trim: Ongoing assessment of the fuel system.
5. Decelerate
Release the accelerator pedal completely and allow the vehicle to coast down to 20 mph (32km/hr). It is crucial not to shift gears, or touch the brake or clutch pedals during deceleration. This gradual slowing down allows for testing of:
- EGR: EGR system diagnostics during deceleration.
- Purge: Evaporative emission system purge diagnostics during deceleration.
- Fuel Trim: Fuel system adjustments during deceleration.
6. Accelerate (Again)
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, ensuring thorough testing of:
- Misfire
- Fuel Trim
- Purge Flow
7. Hold Steady Speed (Extended)
Maintain a steady speed of 55mph (88km/hr) for a longer duration of five minutes. This extended steady speed segment is critical for the:
- Catalyst Monitor: This monitor evaluates the efficiency of the catalytic converter. If your catalyst is marginal or the battery has recently been disconnected, it may require up to five complete driving cycles to accurately determine the catalyst’s condition.
- All diagnostics from step 4 are also performed again (O2 response, Air Intrusive, EGR, Purge, Misfire, and Fuel Trim).
8. Decelerate (Again)
Finally, release the accelerator pedal completely and coast down to 20 mph (32km/hr) without using the brakes, clutch, or shifting gears, just as in step 5. This step again checks:
- EGR
- Purge
- Fuel Trim
By completing these steps, you will have performed a comprehensive GM OBD2 driving cycle, enabling your vehicle’s computer to run all necessary emissions monitors. Remember to consult your vehicle’s repair manual for any specific requirements or variations for your particular GM model.
