Performing a GM OBDII driving cycle is a specific procedure designed to allow your vehicle’s onboard diagnostic system to run all required tests and confirm that the emission control systems are functioning correctly. This process is often necessary after you’ve addressed an issue that triggered a check engine light and cleared the Obd2 Reset Codes. It’s important to understand that simply clearing codes with an OBD2 scanner doesn’t guarantee that your car will pass an emissions test. The driving cycle ensures all emission monitors are set to “ready,” indicating the system has been fully evaluated. Here’s a step-by-step guide to performing a GM OBDII driving cycle:
Step 1: Cold Start – Setting the Initial Conditions
The process begins with a cold start. For the diagnostic cycle to initiate correctly, the engine coolant temperature must be below 122°F (50°C). Crucially, it needs to be within 11°F (6°C) of the ambient air temperature at startup. This condition ensures that the engine management system starts in a specific state, allowing for accurate testing of various components. Avoid inserting the key into the ignition prior to the cold start, as this could prematurely activate the heated oxygen sensor diagnostic and prevent the cycle from running as intended.
Step 2: Idle – Initial System Checks
Once the engine is started cold, allow it to idle for two and a half minutes. During this idle period, it’s beneficial to maximize the electrical load on the engine. Turn on the air conditioner to its maximum setting and engage the rear defroster. Adding electrical load helps to test systems like the O2 sensor heater effectively. This step also initiates diagnostics for Passive Air, Purge “No Flow”, and Misfire detection. Furthermore, if the engine reaches closed loop operation during this idle, the Fuel Trim diagnostics will also begin.
Step 3: Accelerate – Testing Under Load
After the idle period, turn off the air conditioner and all other electrical loads. Now, accelerate using approximately half throttle until you reach a speed of 55mph (88km/hr). This acceleration phase is critical for evaluating Misfire, Fuel Trim, and Purge Flow diagnostics under changing engine load conditions.
Step 4: Hold Steady Speed – Mid-Range Diagnostics
Maintain a constant speed of 55mph (88km/hr) for three minutes. This steady speed segment allows the system to perform diagnostics related to O2 response, air Intrusive, EGR (Exhaust Gas Recirculation), Purge, Misfire, and Fuel Trim. Maintaining a consistent speed is crucial for these monitors to run accurately.
Step 5: Decelerate – Fuel Cut-Off and EGR Testing
Next, release the accelerator pedal completely and allow the vehicle to coast down to 20 mph (32km/hr). It is essential not to shift gears, touch the brake pedal, or depress the clutch during this deceleration phase. This coast-down allows the system to test EGR, Purge, and Fuel Trim diagnostics under fuel cut-off conditions, which is a normal operating mode for modern vehicles during deceleration.
Step 6: Accelerate Again – Re-testing Under Load
Accelerate once more, this time using approximately 3/4 throttle, until you reach a speed between 55-60mph (88-96 km/hr). This second acceleration phase repeats the diagnostics performed in Step 3, again testing Misfire, Fuel Trim, and Purge Flow but under slightly different throttle conditions.
Step 7: Hold Steady Speed (Extended) – Catalyst Monitor
Maintain a steady speed of 55mph (88km/hr) for a longer duration of five minutes. This extended steady speed driving is crucial for the catalyst monitor diagnostics to complete. In addition to the monitors tested in Step 4 (O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim), the catalyst monitor requires a longer period of stable operation to accurately assess catalytic converter efficiency. If the catalyst is near its end of life or if the battery has recently been disconnected, it may take up to five complete driving cycles to definitively determine the catalyst’s state and set the monitor to “ready”.
Step 8: Decelerate (Final) – System Check Completion
Finally, release the accelerator pedal again, and coast down to 20 mph (32km/hr) without using the brakes, clutch, or shifting gears, just as in Step 5. This final deceleration step mirrors the diagnostics performed in Step 5 and helps to ensure all relevant monitors have completed their tests under deceleration conditions.
By diligently following these steps, you are performing a complete GM OBDII driving cycle. After completing this cycle, use an OBD2 scanner to check if all emission monitors are set to “ready”. If they are, and you have addressed the initial issue that triggered the OBD2 reset codes and check engine light, your vehicle should be ready for an emissions test. Remember that this is a GM-specific driving cycle; other manufacturers may have different procedures. Always consult your vehicle’s repair manual for the most accurate driving cycle for your specific make and model.
