EVAP Control System (EVAP Diagnostic Operational Overview)

The Enhanced Evaporative System Diagnostic is a precise series of five operational sub-tests on the system that takes about 25 seconds to run. In some cases, the failure of only one sub-test will end all evaporative system testing. However, in many cases, the on-board system will conduct a series of sub-tests before storing any DTCs. A brief description of each of the sub-tests follows.

Phase A
Purge valve is closed at a constant rate. Of the fuel tank pressure does not rise to a calibrated value within a set time, the vent valve is blocked and code P0440 will set. If the tank pressure is not steady, code P0451 will set.

Phase B
Purge valve is also closed and pressure due to fuel evaporation is allowed to build up. The diagnostic is terminated if the pressure become too high. If the pressure fall too low, the purge is blocked open, and code P1441 will set.

Phase C
The vent valve is open, and the purge valve is open 20% over a period of 4 seconds. If the pressure falls to a calibrated value within a set time, the vent is stuck closed, and code P0440 will set. If the pressure does not fall to this value, the diagnostic moves to phase D.

Phase D
Close vent value. If the pressure fall to quickly, a large leak is detected and code P0455 will set. If the pressure falls to a calibrated value before the time for this phase is expired, the purge is stuck open, and code P1441 will set.

Phase E
Close purge value. If the pressure does not increase by a calibrated value, a small leak is detected, and code P0442 is set.