The evaporative emission (EVAP) control system limits the fuel vapors from escaping into the atmosphere. The EVAP transfers the fuel vapor from the sealed fuel tank to an activated charcoal EVAP canister. The EVAP canister stores the vapors until the engine is able to use the extra fuel vapor.

When the engine is able to use the extra fuel vapor, the intake air flow purges the fuel vapor from the carbon element, and then the normal combustion process consumes the fuel vapor.

The system is required in order to detect the evaporative fuel system leaks as small as 0.040 between the fuel filler cap and the purge solenoid. The system can test the EVAP system integrity by applying a vacuum signal (ported or manifold) to the fuel tank in order to create a small vacuum.

The control module then monitors the ability of the system to maintain the vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks, and a pass report is sent to the control module. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts up to 8 specific sub-tests in order to detect the fault conditions. If the diagnostic fails a sub-test, the control module stores a diagnostic trouble code (DTC) in order to indicate the type of fault detected.