Ensuring your vehicle is ready for an emissions test, commonly known as a smog check, often requires completing an OBD2 drive cycle. This process allows your car’s computer to run self-tests on various emission control systems, including the Evaporative Emission Control (EVAP) system. For General Motors (GM) vehicles like Chevrolet, Pontiac, Buick, and Cadillac, performing a specific drive cycle is crucial, especially when dealing with EVAP monitor readiness. This guide provides a detailed, step-by-step approach to execute a GM OBD2 drive cycle, focusing on completing the EVAP monitor.
Understanding the GM OBD2 Drive Cycle and EVAP Monitor
The On-Board Diagnostics II (OBD2) system in your GM vehicle monitors the performance of emission-related components. After a battery disconnection or fault code clearing, these monitors may reset to an “not ready” state. To achieve a “ready” status, the vehicle needs to complete a drive cycle, which is a series of specific driving conditions that prompt the computer to run diagnostic tests.
The EVAP system is designed to prevent fuel vapors from escaping into the atmosphere. The EVAP monitor is often one of the most challenging to get into a “ready” state because it has stringent testing conditions. A properly executed GM OBD2 drive cycle, specifically targeting the EVAP monitor, is essential for passing smog checks and ensuring your vehicle is environmentally compliant.
Performing the General GM OBD2 Drive Cycle
This drive cycle is designed to run most of the emission monitors on your GM vehicle, including EVAP. Follow these steps carefully for optimal results:
1. Cold Start Conditions:
- Begin from a Cold Start: The engine must be completely cold. Ensure the vehicle has been off for at least one hour.
- Fuel Level: Your fuel tank should be between 1/4 and 3/4 full.
- Coolant Temperature: Engine coolant temperature must be below 122°F (50°C) and within 11°F (6°C) of the ambient air temperature.
- Key Position: Do not turn the key to the “ON” position before starting, as this may prevent the heated oxygen sensor (O2) diagnostic from running.
2. Initial Idle:
- Start the Engine: Start your GM vehicle without pressing the accelerator pedal.
- Idle for 2.5 Minutes: Let the engine idle for 2 minutes and 30 seconds. During this period, turn on the air conditioner and rear defroster to increase engine load. This idle time allows the ECU (Engine Control Unit) to run several self-tests, including:
- Oxygen Sensor Heater self-test
- Air Injection System Passive Air test
- EVAP Purge “No Flow” test
- Misfire Detection
3. Acceleration Phase 1:
- Turn off Accessories: Switch off the air conditioner and rear defroster.
- Accelerate to 55 mph (88 km/h): Begin driving and apply approximately half throttle as soon as it is safe to do so. Accelerate until you reach 55 mph. During this acceleration, the ECU will perform self-tests on:
- Misfire Detection
- Fuel Trim
- EVAP Purge Flow
4. Steady Speed Phase 1:
- Maintain 55 mph for 3 Minutes: Hold a steady speed of 55 mph for three minutes. During this phase, the following diagnostics are performed:
- Oxygen Sensor (O2) response
- Air Intrusive System (AIS)
- EGR Flow
- Purge Valve
- Misfire Detection
- Fuel Trim
5. Deceleration Phase 1:
- Coast Down to 20 mph (32 km/h): Slowly release the accelerator pedal and allow your vehicle to coast down to 20 mph. Do not use the brakes or shift gears to slow down. This deceleration allows the ECU to self-test:
- EGR System
- EVAP Purge System
- Long Term Fuel Trims
6. Acceleration Phase 2:
- Accelerate to 55-60 mph (88-96 km/h): Accelerate again, this time using about 3/4 throttle, until you reach 55-60 mph. This step repeats the same self-tests as the first acceleration phase (Step 3).
7. Steady Speed Phase 2:
- Maintain 55 mph for 5 Minutes: Hold a steady speed of 55 mph for five minutes. In addition to the tests from the first steady speed phase (Step 4), the ECU will now perform:
- Catalyst monitor diagnostics. If the catalytic converter is marginal or the battery has been recently disconnected, it may take up to 5 complete drive cycles to complete the CAT monitor.
8. Deceleration Phase 2:
- Coast Down to 20 mph (32 km/h): Repeat the deceleration process from Step 5. Slowly release the accelerator and coast down to 20 mph without using brakes or shifting gears. This performs the same diagnostics as in the first deceleration phase.
This completes one GM OBD2 drive cycle. Performing this drive cycle twice is generally recommended to ensure all monitors, including the EVAP monitor, have a chance to run and complete, preparing your vehicle for a smog check.
Completing the EVAP Monitor Individually
If you specifically need to complete only the EVAP monitor, or if it remains incomplete after the general drive cycle, you can try this specific procedure:
EVAP Monitor Drive Cycle:
Prerequisite:
- No trouble codes present.
- Fuel tank between 1/4 and 3/4 full.
Steps:
- Drive at Steady Speed: Drive your GM vehicle for 5 minutes at a steady speed between 30 and 60 mph (48-96 km/h).
- Idle: Stop your vehicle and let the engine idle for 4 minutes.
- Engine Off: Turn off the engine and remove the ignition key.
The EVAP monitor test is often initiated and completed after the engine is shut off and the ignition is turned off. It may take several drive cycles and overnight periods for the EVAP monitor to complete, as it often runs under specific conditions related to temperature and pressure changes within the fuel system.
Completing Other Emission Monitors Individually (Reference)
For completeness, here are the procedures to target other specific monitors if needed.
Catalyst Monitor (CAT):
Prerequisites:
- No trouble codes present.
- Intake Air Temperature (IAT) above 32°F (0°C).
- Engine Coolant Temperature (ECT) above 167°F (75°C).
- Mass Air Flow (MAF) sensor reading greater than 15 gm/s.
- Engine load below 63% and steady.
- Engine speed below 3000 RPM.
Steps:
- Idle for 5 Minutes: Idle your vehicle for 5 minutes to achieve closed loop operation.
- Drive at 55+ mph for 5 Minutes: Drive at a steady speed above 55 mph for 5 minutes.
- Drive at 40 mph for 2 Minutes: Drive at a steady 40 mph for 2 minutes.
- Drive at 55 mph for 2 Minutes: Drive at a steady 55 mph for 2 minutes.
EGR Monitor (EGR):
Prerequisites:
- No trouble codes present.
- Engine Coolant Temperature (ECT) above 75°F (24°C).
- Barometric pressure (BARO) above 72kPa.
Steps:
- Idle for 5 Minutes: Idle your vehicle for 5 minutes to achieve closed loop operation.
- Accelerate and Drive at 50+ mph for 1 Minute: Accelerate and drive at a steady speed above 50 mph for 1 minute.
- Decelerate to 20 mph: Slow down to 20 mph without using brakes, clutch, or shifting gears.
- Repeat Steps 2 & 3 Five Times: Repeat steps 2 and 3 a total of 5 times.
O2 Sensor Monitor (O2):
Prerequisites:
- No trouble codes present.
- Intake Air Temperature (IAT) above 32°F (0°C).
- Engine Coolant Temperature (ECT) above 167°F (75°C).
Steps:
- Idle for 4 Minutes: Idle your vehicle for 4 minutes to achieve closed loop operation.
- Drive at 50+ mph for 3 Minutes: Drive at a steady speed above 50 mph for 3 minutes.
- Drive at 40 mph for 2 Minutes: Drive at a steady speed at 40 mph for 2 minutes.
- Drive at 50 mph for 2 Minutes: Drive at a steady speed at 50 mph for 2 minutes.
Tips for a Successful GM OBD2 Drive Cycle
- Follow Instructions Precisely: Adhere to the speed and time durations as closely as possible.
- Avoid Aggressive Driving: Drive smoothly and avoid sudden acceleration or braking, except where instructed.
- Monitor Engine Temperature: Ensure your engine reaches operating temperature before starting the drive cycle.
- Use a Scan Tool: An OBD2 scan tool can help monitor your engine parameters and check monitor status in real-time.
- Be Patient: Some monitors, especially EVAP and Catalyst, may take multiple drive cycles to complete.
- Check for Trouble Codes: Ensure there are no pending or active trouble codes before starting the drive cycle, as they can prevent monitors from running.
By following these detailed steps and tips, you can effectively perform a GM OBD2 drive cycle and increase the chances of completing the EVAP monitor and other emission monitors, ensuring your GM vehicle is ready for its next smog inspection. Remember to consult your vehicle’s service manual for any specific recommendations or variations for your particular model.
