During specific operating conditions, the engine control module (ECM) performs various tests on the evaporative emission system. The system tests consist of the following series of events. After ensuring that the evaporative emission (EVAP) purge solenoid valve duty cycle has dropped to 0 percent, indicating that the valve is closed, the ECM commands the EVAP canister vent solenoid valve closed, sealing the system. The ECM monitors the accumulation of vapor pressure within the fuel tank via the fuel tank pressure (FTP) sensor. The EVAP test is aborted if the vapor pressure is too high. If any vapor pressure is measured, the pressure will be used later in order to compensate for a pressure reading during the small leak detection test. If a vacuum is measured which exceeds a calculated limit during vapor accumulation, diagnostic trouble code (DTC) P0440 will set. The EVAP canister purge solenoid valve is then opened. Simultaneously, the vent solenoid is opened. If the vapor pressure does not bleed off or bleeds off too slowly, DTC P0440 is set. Once the EVAP purge solenoid valve reached the desired position for the remainder of the diagnostic test, the vent solenoid will be closed again. This causes a vacuum to be applied to the entire EVAP system. The ECM monitors the vacuum level within the system. If the desired vacuum level cannot be achieved, or if the vacuum level is reached but took too much time, DTC P0455 will set. Once the desired vacuum level is reached, the purge solenoid is closed, sealing the system. The ECM continues to monitor the fuel tank pressure sensor signal, measuring the rate of vacuum decay. If the system holds vacuum, the vent solenoid is opened and the test is completed. In addition to the system tests, the ECM monitors the circuit integrity of the purge solenoid (DTC P0443), the vent solenoid (DTC P0446) and the fuel tank pressure sensor (DTC P0450).

Diagnostic test has run to completion and determined that this DTC was detected.

If an intermittent problem is suspected, check for these possible causes:

Refer to Symptoms for Intermittents.

Number(s) below refer to the step number(s) on the Diagnostic Table.

3. Inspect the Purge valve and vacuum lines for traces of carbon. Carbon escaping from the canister may prevent the valve from closing properly. If you find this condition, disconnect both ends of the purge line and blow out any contaminants using the pressurized gas from the EVAP pressure/purge cart. Replace the Purge valve and the canister.