Performing a complete driving cycle is crucial for diagnosing emission-related issues on your GM vehicle. This process allows the OBD2 system to run various tests, known as monitors, and determine if all components are functioning correctly. Knowing how to check these monitors can help you identify potential problems before they become major headaches.
Understanding OBD2 Monitors on GM Vehicles
The On-Board Diagnostics II (OBD2) system in your GM vehicle continuously monitors various emission-related components. Each component has a specific monitor that runs tests to ensure it’s working within acceptable parameters. These tests are essential for passing emissions inspections and maintaining optimal vehicle performance. Completing a driving cycle allows these monitors to run and store their results, which can then be read using an OBD2 scanner.
The GM Driving Cycle: A Step-by-Step Guide
A GM driving cycle typically takes about 15 minutes to complete and should be performed under specific conditions to ensure accurate results. Here’s a comprehensive guide:
1. Cold Start
- Requirement: Engine coolant temperature below 50°C (122°F) and within 6°C (11°F) of ambient temperature. Avoid turning the key on before starting to prevent interference with the heated oxygen sensor diagnostic.
- Purpose: Enables tests for the O2 heater, Passive Air, Purge “No Flow,” and Misfire systems.
2. Idle
- Requirement: Run the engine for 2.5 minutes with the AC and rear defroster on. Maximize electrical load for optimal results.
- Purpose: Tests O2 heater, Passive Air, Purge “No Flow,” Misfire, and Fuel Trim (if closed loop is achieved).
3. Acceleration
- Requirement: Turn off AC and all loads. Accelerate at half throttle to 88 km/h (55 mph).
- Purpose: Diagnoses Misfire, Fuel Trim, and Purge Flow systems.
4. Hold Steady Speed
- Requirement: Maintain 88 km/h (55 mph) for 3 minutes.
- Purpose: Tests O2 response, Air Intrusive, EGR, Purge, Misfire, and Fuel Trim.
5. Deceleration
- Requirement: Decelerate to 32 km/h (20 mph) without braking, shifting, or using the clutch. Let the vehicle coast.
- Purpose: Diagnoses EGR, Purge, and Fuel Trim systems.
6. Acceleration
- Requirement: Accelerate at 3/4 throttle to 88-96 km/h (55-60 mph).
- Purpose: Similar diagnostics as Step 3.
7. Hold Steady Speed
- Requirement: Maintain 88 km/h (55 mph) for 5 minutes.
- Purpose: Performs all diagnostics from Step 4, plus crucial catalyst monitor diagnostics. Note: A marginal catalyst or disconnected battery may require up to 5 cycles for accurate results.
8. Deceleration
- Requirement: Decelerate to a stop without braking, shifting, or using the clutch. Let the vehicle coast.
- Purpose: Similar diagnostics as Step 5.
Checking Monitor Status with an OBD2 Scanner
After completing the driving cycle, connect an OBD2 scanner to your vehicle’s diagnostic port. The scanner will display the status of each monitor, indicating whether it has run and passed or failed its tests. This information is crucial for pinpointing the source of any emission-related problems.
Conclusion
Understanding how to check monitors on your OBD2 GM vehicle is essential for maintaining its health and ensuring it passes emissions tests. By following the steps outlined in this guide, you can perform a complete driving cycle and accurately assess the status of your vehicle’s emission system. Remember to consult your vehicle’s specific repair manual for any variations in the driving cycle procedure.
