Understanding and Completing the OBD2 EVAP Monitor Drive Cycle

The OBD2 Evaporative Emission (EVAP) monitor is a critical component of your vehicle’s emission control system. It’s designed to detect leaks in the fuel vapor recovery system, preventing harmful hydrocarbons from escaping into the atmosphere. For vehicle owners, especially those with GM vehicles, getting the EVAP monitor to show a “Ready” status can sometimes be a frustrating hurdle, particularly when trying to pass an emissions test. This article provides a comprehensive guide to understanding and completing the Obd2 Evap Monitor drive cycle, focusing on the General Motors (GM) procedure, to ensure your vehicle is emissions compliant.

What is the OBD2 EVAP Monitor?

The Evaporative Emission Control System (EVAP) monitor is part of your car’s onboard diagnostic system (OBD2). Its primary function is to monitor the integrity of the EVAP system. This system prevents fuel vapors from escaping the gas tank and fuel system into the atmosphere. The EVAP system typically includes components like the fuel tank, fuel lines, vapor canister, purge valve, and various sensors.

The OBD2 system runs self-tests on the EVAP system, and the EVAP monitor is responsible for indicating whether these tests have been completed and passed. When the EVAP monitor status is “Ready” or “Complete,” it signifies that the system has been tested and no issues were detected. However, if the monitor status is “Not Ready” or “Incomplete,” it means the tests haven’t run or haven’t been successfully completed. This “Not Ready” status can prevent your vehicle from passing an emissions test, even if there isn’t an active fault code.

Why is the EVAP Monitor Not Ready?

Several reasons can cause the OBD2 EVAP monitor to be in a “Not Ready” state:

Recent Battery Disconnection or Code Reset: Clearing diagnostic trouble codes (DTCs) or disconnecting the vehicle’s battery will reset all OBD2 monitors to “Not Ready.” After a reset, the vehicle needs to complete specific drive cycles for each monitor to run its self-tests and achieve a “Ready” status.
Interrupted Drive Cycles: The EVAP monitor, like other OBD2 monitors, requires specific driving conditions to run its tests. If these conditions are not consistently met, or if the drive cycle is interrupted, the monitor may not complete.
Complex Enabling Criteria: The EVAP monitor often has more complex and stringent enabling criteria compared to other monitors. This can include factors like ambient temperature, coolant temperature, intake air temperature, barometric pressure, fuel level, and even fuel alcohol content.
Underlying EVAP System Issues: While the monitor itself might be functioning correctly, a genuine issue within the EVAP system (like a small leak) can prevent the monitor from setting to “Ready.” In such cases, the system may be detecting a problem, even if it’s not severe enough to trigger a fault code immediately.
GM Specific Complexity: As noted in the original article, GM vehicles are known to be particularly sensitive and have a more complex EVAP monitor procedure. This means it can take more time and specific driving patterns to get the monitor to complete on GM cars and trucks.

GM EVAP Monitor Drive Cycle Procedure: Step-by-Step Guide

The following procedure outlines the GM Inspection/Maintenance (I/M) Complete System Set Procedure, specifically adapted for the EVAP monitor. It is designed to create the necessary conditions for the EVAP monitor to run and complete its self-test. It’s crucial to follow these steps precisely and ensure all pre-conditions are met.

Pre-Conditions for a Cold Start

Before starting the drive cycle, verify that the following conditions are met. These “cold start” conditions are essential for initiating the EVAP monitor test:

Voltage: Battery voltage between 11-18 volts.
Barometric Pressure (BARO): Greater than 75 kPa.
Engine Coolant Temperature (ECT): Startup ECT between 4-30°C (39-86°F).
Intake Air Temperature (IAT): Startup IAT between 4-30°C (39-86°F).
Temperature Difference: Difference between IAT and ECT less than or equal to 6°C (10.8°F).
Ambient Air Temperature: Between 4-30°C (39-86°F).
Fuel Level: Between 15-85 percent of fuel tank capacity.
Fuel Alcohol Content:
- Without RPO LMG (Regular Fuel): Less than 15 percent.
- With RPO LMG (Flex Fuel): Less than 87 percent.

Important Preparation:

Cold Soak: Ensure the vehicle has been sitting with the ignition OFF for at least 5 hours, ideally overnight, to ensure a true cold start. For O2S Heater Monitor readiness, it’s recommended to ensure the ignition has been off for at least 10 hours if that monitor is also not ready.
Scan Tool (Optional but Recommended): Using an OBD2 scan tool to monitor the I/M System Status and the I/M monitor-enabled parameters can be very helpful to confirm conditions and monitor progress.

Step-by-Step Procedure

Initial Setup:
- Park the vehicle on a level surface and apply the parking brake.
- For automatic transmissions, ensure the vehicle is in Park. For manual transmissions, place it in Neutral.
- Turn OFF all accessories: headlights, radio, HVAC system, and any aftermarket electrical equipment.
Cold Start and Idle:
- Turn the ignition ON but do not start the engine yet. Leave the ignition ON for approximately 5 minutes to allow the heated oxygen sensors (HO2S) to cool down (this is crucial as per the original procedure).
- Start the engine. Do not turn the engine OFF again for the rest of the procedure.
- Idle the engine for at least 2 minutes until the engine coolant temperature reaches 65°C (149°F).
Low Speed Cruise:
- Drive the vehicle under the following conditions for 6.5 minutes:
  - Mass Air Flow (MAF) parameter between 4-30 g/s.
  - Engine speed steady between 1000-3000 RPM.
  - Maintain a steady speed, avoiding sudden acceleration or deceleration.
Idle Again:
- Return the vehicle to idle for 1 minute. Keep the vehicle in Park (automatic) or Neutral (manual).
Low Speed, Low Load Drive (Drive or Clutch In):
- Apply and hold the brake pedal.
- Shift to Drive (automatic) or depress the clutch pedal (manual).
- Operate the vehicle under the following conditions for 2 minutes:
  - Depress the accelerator pedal slightly until the Throttle Position (TP) Sensor angle is more than 2 percent.
  - MAF signal between 15-30 g/s.
  - RPM steady between 1200-2000 RPM.
- Important: Do not touch the accelerator pedal during the idle period before this step. Any change in TP Sensor angle or engine speed can invalidate this part of the test.
Idle After Low Load Drive:
- Release the accelerator pedal.
- Shift the vehicle to Park (automatic) or Neutral and release the clutch pedal (manual).
- Allow the engine to idle for 2 minutes.
Quick Throttle Bursts:
- Quickly depress the accelerator pedal until the TP Sensor Angle is more than 8 percent and then immediately return to idle.
- Repeat this quick throttle burst and return to idle sequence 3 times.
Idle After Throttle Bursts:
- Allow the engine to idle for at least 2 minutes.
Very Low Speed Drive:
- Release the parking brake.
- Drive the vehicle at a very slow speed, 24 km/h (15 mph) or slower, for 2 minutes.
Highway Speed Cruise and Deceleration:
- Continue driving for at least 5.5 miles (8.8 km) between 45-112 km/h (28-70 mph), ensuring the vehicle reaches at least 80 km/h (50 mph) at some point during this segment.
Deceleration Fuel Cut-Off:
- Release the accelerator pedal completely for at least 2 seconds. This allows the vehicle to enter deceleration fuel cut-off, which is important for the EVAP test.
Moderate Acceleration and Cruise:
- Depress the accelerator pedal until the TP Sensor angle increases by 3-20 percent.
- Maintain a safe and steady speed for 1 minute.
Final Idle in Drive/Neutral:
- Safely stop the vehicle.
- Keep the engine running in Drive (automatic) or Neutral with the clutch pedal depressed and parking brake applied (manual) for 2 minutes.
Final Idle in Park/Neutral:
- Shift to Park (automatic) and apply the parking brake, or Neutral and release the clutch pedal (manual).
- Allow the engine to idle for 2 minutes.
Cool Down Period (Critical):
- Turn OFF the ignition.
- Exit the vehicle and do not disturb the vehicle in any way for 45 minutes. Disturbing the vehicle can invalidate the test.
Verification:
- After the 45-minute cool-down, turn the ignition ON and use an OBD2 scan tool to check the Inspection/Maintenance (I/M) System Status.
- All I/M System Status indicators, including EVAP, should now display “YES” or “Ready.”
If the EVAP I/M System Status indicator still displays “NO”:
- Turn OFF the ignition for a full 17 hours and repeat steps 1-18 up to six more times.
- Ensure all cold start conditions are met before each attempt.
- If the EVAP monitor still does not set to “Ready” after multiple attempts, there may be an underlying issue within the EVAP system. Consult a repair manual or a qualified mechanic for further diagnosis and repair. Check for any pending or stored DTCs related to the EVAP system.

Troubleshooting Tips for OBD2 EVAP Monitor Readiness

Patience is Key: The EVAP monitor drive cycle can be time-consuming and may require multiple attempts, especially on GM vehicles. Don’t get discouraged if it doesn’t set immediately.
Consistent Conditions: Ensure that you consistently meet the pre-conditions and driving parameters throughout the drive cycle.
Avoid Interruptions: Do not interrupt the drive cycle by turning off the engine or making unnecessary stops unless instructed in the procedure.
Check for DTCs: Use an OBD2 scanner to check for any pending or stored Diagnostic Trouble Codes (DTCs). EVAP system codes (P0440-P0460 range) can prevent the monitor from setting to “Ready.” Address any DTCs before attempting the drive cycle again.
Fuel Cap: A loose or faulty fuel cap is a common cause of EVAP system leaks. Ensure your fuel cap is properly tightened and in good condition.
Professional Help: If you’ve attempted the drive cycle multiple times and the EVAP monitor still won’t set, it’s advisable to seek professional help from a qualified mechanic. They can perform more in-depth diagnostics of the EVAP system.

Conclusion

Successfully completing the OBD2 EVAP monitor drive cycle, particularly on GM vehicles, requires patience and attention to detail. By understanding the process, adhering to the pre-conditions, and carefully following the step-by-step procedure, you can increase your chances of setting the EVAP monitor to “Ready” and ensuring your vehicle passes its emissions test. Remember to use an OBD2 scanner to monitor your progress and troubleshoot any potential issues. If problems persist, don’t hesitate to consult a professional for expert diagnosis and repair of your vehicle’s EVAP system.