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That could be a loose, missing or a bad gas cap, most times it is. However, it could be a number of other thing as well. Here is the test.
The Powertrain Control Module (PCM) supplies a ground to energize the valve (purge ON). The EVAP purge valve control is Pulse Width Modulated (PWM) or turned ON and OFF several times a second. The duty cycle (pulse width) is determined by engine operating conditions including load, throttle position, coolant temperature and ambient temperature. The duty cycle is calculated by the PCM and the output is commanded when the appropriate conditions have been met. The system checks for conditions that cause the EVAP system to purge continuously by commanding the EVAP canister vent valve ON and the EVAP purge valve OFF (EVAP canister vent valve CLOSED, EVAP purge PWM 0%). If vacuum level in the fuel tank increases during the test, a continuous purge flow condition is indicated. This can be caused by any of the following conditions:
CONDITIONS FOR SETTING THE DTC
ACTION TAKEN WHEN THE DTC SETS
NOTE: Although these diagnostics are considered type A, they act like type B diagnostics under certain conditions. Whenever the EVAP diagnostics report that the system has passed, or if the battery has been disconnected, the diagnostic must fail during two consecutive cold start trips before setting a DTC. The initial failure is not reported to the diagnostic executive or displayed on a scan tool. A passing system always reports to the diagnostic executive immediately. CONDITIONS FOR CLEARING THE MIL/DTC
DIAGNOSTIC AIDS Check for the following conditions:
Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
Inspect the wiring harness for damage.
Verify that the source vacuum line routing to the EVAP purge valve is correct and that the EVAP purge and source vacuum lines to the EVAP purge solenoid are not switched.
A malfunctioning canister may intermittently allow charcoal into the EVAP purge solenoid, vacuum switch, and associated lines causing a DTC to be set. Use the following procedure to check for a carbon release condition:
Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to set occurs. This may assist in diagnosing the condition. TEST DESCRIPTION Number(s) below refer to the step number(s) on the Diagnostic Table.