Operation of Diagnostic Routine

Basically, on vehicles with enhanced EVAP, the ECM has the ability to check the integrity of the EVAP system once during each "trip". Under a set of very exact operating conditions (called "enable criteria"), the ECM can check for both a "large leak" and a "small leak". Here is a quick review of what happens during the diagnostic procedure.

When the enable criteria are met, the ECM will energize the "canister close valve", increase the duty cycle of the EVAP valve, and look for a small pressure drop (vacuum) to be formed at the pressure sensor in the fuel tank. If the ECM does not see a small pressure drop within several seconds, the ECM determines that there is a "large leak" and DTC P0455 is set.


If the ECM determines that an adequate "pressure drop" (vacuum) has occurred in the first phase, then the ECM shuts off the EVAP valve, but holds the canister close valve closed for several seconds. During this phase, if the pres sure rises appreciably, the ECM determines that there is a "small leak" and DTC P0442 is set.


When either of these DTC's is found, the service manual test procedure calls for a thorough check for leaks in the EVAP system. A leak as small as 1 mm (0.040") can cause P0442 to occur. It is important, therefore, that you are very careful when performing the functions on the pages that follow.