Ensuring your vehicle is running optimally and passing emissions tests often requires more than just basic maintenance. For General Motors vehicles, a specific procedure known as the OBDII drive cycle is crucial for resetting and testing onboard diagnostic systems. This process allows your vehicle’s computer to run comprehensive diagnostics, ensuring all systems are functioning correctly, especially after repairs or when addressing a “check engine light” issue. Utilizing an OBDII scanner can be invaluable in monitoring this process, confirming readiness codes are set, and diagnosing any persistent problems.
The General Motors driving cycle is designed to execute diagnostics on all vehicle systems within a relatively short period, typically under fifteen minutes. Following this cycle meticulously is key to a successful diagnostic evaluation. Here’s a step-by-step guide to performing the GM OBDII drive cycle effectively:
1. Cold Start:
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 at startup. It’s important not to turn 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:
Start the engine and let it idle for two and a half minutes. During this idle period, turn on the air conditioner and the rear defroster to increase the electrical load on the engine. Maximizing the electrical load during idle helps to thoroughly test components such as the O2 sensor heater, passive air system, purge valve for “no flow” conditions, and misfire detection. If closed-loop operation is achieved, this step also assesses fuel trim.
3. Accelerate:
Turn off the air conditioner and all other electrical loads. Accelerate smoothly at half throttle until you reach a speed of 55 mph (88 km/hr). This acceleration phase is critical for activating diagnostics related to misfire, fuel trim adjustments, and purge flow within the evaporative emissions control system.
4. Hold Steady Speed:
Maintain a constant speed of 55 mph (88 km/hr) for three minutes. Keeping a steady speed allows the OBDII system to evaluate the O2 sensor response time, intrusive air system, exhaust gas recirculation (EGR) functionality, purge system efficiency, misfire detection consistency, and ongoing fuel trim adjustments.
5. Decelerate:
Release the accelerator pedal and allow the vehicle to coast down to 20 mph (32 km/hr). It’s crucial during deceleration not to shift gears or apply the brakes or clutch. This coast-down period is essential for diagnosing the EGR system, purge flow, and fuel trim under changing engine load conditions.
6. Accelerate Again:
Accelerate once more, this time at 3/4 throttle, until reaching a speed between 55-60 mph (88-96 km/hr). This second acceleration phase repeats the diagnostic checks performed in step 3, further validating the systems under slightly higher load conditions.
7. Hold Steady Speed Again:
Maintain a steady speed of 55 mph (88 km/hr) for a longer duration of five minutes. In addition to the diagnostics performed in step 4, this extended steady-speed segment is crucial for evaluating the catalyst monitor. If the catalytic converter’s performance is marginal or if the vehicle battery has been recently disconnected, it might require up to five complete driving cycles to accurately assess the catalyst’s state.
8. Decelerate Again:
Finally, release the accelerator pedal and allow the vehicle to coast down without shifting, braking, or clutching, similar to step 5. This deceleration phase again assesses the EGR, purge, and fuel trim systems as the vehicle slows down.
Successfully completing the GM OBDII drive cycle, ideally while monitoring with an OBDII scanner, ensures that your vehicle’s diagnostic systems are thoroughly tested. This procedure is invaluable for verifying repairs, confirming system readiness for emissions testing, and effectively troubleshooting engine-related issues indicated by a check engine light. Remember to always prioritize safety and adhere to local traffic laws while performing this driving cycle.
