EVAP Diagnostic Operational Overview

The evaporative system diagnostic is a series of five operational tests, which take approximately 25 seconds to run. These tests run in sequence. If any one test fails, the entire diagnostic will end. This will cause a DTC to set. The following is a description of the five tests:

Phase A - The purge valve is closed. If vacuum is too high after the purge valve is closed, the vent valve is stuck closed and DTC P0440 - EVAP System - will set. The vent valve should allow fresh air flow into the system causing fuel tank pressure to be the same, or near, atmospheric pressure. If fuel tank pressure is not steady at this point, DTC P0451 - EVAP Fuel Tank Pressure Sensor Circuit Performance - will set.

Phase B - The vent valve is closed and the pressure increase due to fuel evaporation is monitored. This diagnostic will be terminated if pressure increases above an allowable level. If after a certain amount of time, the built up pressure is below a set limit, the purge valve is stuck open and DTC P1441 EVAP System Stall Leak Detected - is set.

Phase C - The vent valve is opened completely and the purge valve is opened to 20% over a period of four seconds. This should not pull a significant vacuum on the fuel tank. If it does pull a significant vacuum, then sufficient air is not entering the system through the vent valve. This means the vent valve is stuck closed and DTC P0440 - EVAP System - is set.

Phase D - The vent valve is closed to allow vacuum build up for leak detection. If a rich mixture is detected, then the EVAP canister is loaded with fuel vapor, possibly caused by a recent fuel fill, and the diagnostic will be aborted. If the system fails to reach a certain level of vacuum within a set time, a large leak is detected, DTC P0455 - EVAP System Gross Leak Detected - is set, and the diagnostic will end. DTC P0455 could be caused by a vent valve stuck open, a purge valve stuck closed, or some other large leak. If too much vacuum is created, the purge is stuck open and DTC P1441 - EVAP System Flow During Non-Purge - will set. If vacuum becomes too high, the purge valve will be closed.

Phase E - Both the EVAP vent and purge valves are closed. The vacuum of the system should remain nearly constant. If the ECM detects a loss of vacuum over a set amount of time, a small leak is detected and DTC P0442 - EVAP System Small Leak Detected - will set. The EVAP diagnostic is now completed.