Circuit Description

The Evaporative Emission (EVAP) system is used to store fuel vapors to reduce the amount of unburned fuel from escaping into the atmosphere. The EVAP system consists of the EVAP canister, the Fuel Tank Pressure (FTP) sensor, the EVAP lines and hoses, the EVAP purge solenoid which is normally closed, the EVAP service port, the EVAP vent solenoid which is normally open, the fuel tank, and the Engine Control Module (ECM). The ECM monitors the EVAP system for circuit faults in the FTP sensor, the EVAP purge solenoid and the EVAP vent solenoid circuits. The ECM also monitors the EVAP system for small and large leaks. During the DTC diagnostic, the ECM monitors the FTP sensor for an atmospheric pressure reading when all of the DTC parameters have been met with the engine running. The ECM will then command the EVAP vent solenoid ON, closed valve, and command the EVAP purge solenoid to a fixed duty cycle. The ECM will let the pressure drop in the fuel tank for a certain length of time, after which it will turn OFF the EVAP purge solenoid. If a correct amount of vacuum is achieved, the ECM will run the DTC P0442 diagnostic, which will monitor the decay in the vacuum of the sealed system over a calibrated amount of time. DTC P0442 sets when the vacuum decay is greater than the calibrated amount for a certain length of time due to a small leak.