OBD II Readiness Monitors

Non-Continuous Monitors

EVAP System Monitor

The EVAP system monitor allows the PCM to test the integrity of the fuel system and its ability to draw fuel vapor into the engine for combustion. A portion of the monitor is activated during canister purge operation that occurs under normal engine and vehicle operating conditions. The leak detection portion of the monitor may be activated under normal engine and vehicle operating conditions, during extended idle or soon after the vehicle has been shut off. As with most other monitored OBD II systems, all EVAP electrical components are checked for electrical and rationality faults during continuous monitoring.

The EVAP system monitor checks for fuel vapor leaks by performing either a pressure or vacuum test on the fuel system. For 1996 through 1999 vehicles, the federal standard allows leaks up to the equivalent of a hole .040 inches in diameter in a fuel vapor hose or filler cap. For 2000 and newer vehicles, the leakage rate has been reduced to the equivalent of a .020 in. diameter hole. The EVAP monitor checks for fuel vapor leaks (including a loose or missing gas cap).

During EVAP system monitoring the PCM tests for vapor leaks by applying vacuum or pressure to the fuel tank, vapor lines and charcoal canister. If the PCM detects no airflow when the EVAP canister purge valve is opened, or it detects a leak that equals or exceeds the amount of air that would pass through a hole 0.040 inches in diameter (0.020 inches for 2000 and up model year vehicles) a pending DTC fault code is stored. The PCM will command the check engine light to illuminate and store a DTC if the fault occurs during 2 drive cycles.

The requirements for running the EVAP monitor vary depending on the year, make and model of your vehicle. Generally speaking, the fuel tank must be 1/4 to 3/4 full because a near empty tank or a full tank can affect the accuracy of the EVAP self-test. The ambient outside temperature must not be too hot (above 95 degrees) or too cold (below 30 degrees) because this affects fuel volatility and the amount of vapor inside the tank. The vehicle must also have sat overnight or for at least 8 hours without being driven. On some vehicles, the EVAP monitor runs when the vehicle has been cruising on the highway at light throttle at a speed of 45 to 65 mph for at least 10 minutes or more. All of the components in the EVAP control system must also be functioning normally. The presence of any EVAP related DTC fault codes will prevent the EVAP System monitor from running.