Monitor Description

The ECM tests the evaporative emissions (EVAP) system using the fuel tank pressure sensor, the canister close valve (CCV), and the VSV for EVAP. The ECM closes the EVAP system and introduces a negative pressure (vacuum) into it. The ECM then monitors the internal pressure using the fuel tank pressure sensor. (Refer to Figure)

A leak in the evaporative emission system prompts the ECM to set DTC P0442, P0456.

The ECM checks for leaks in the system by introducing a high negative pressure from the intake manifold by commanding the VSV for EVAP to open while the VSV for CCV (vent) is closed. After sufficient time has elapsed the fuel tank should have developed a high negative pressure and the VSV for EVAP is closed. The ECM then monitors the pressure-rise (loss of vacuum) in the fuel tank. If the pressure rises too rapidly, the ECM concludes that there is a leak in the system. The ECM will turn on the MIL and a DTC is set. The ECM has separate DTCs for small and large leaks.

1. Large increase of the internal pressure
2. Slight increase of the internal pressure

Condition (a) represents leakage somewhere in the evaporative emission system and DTC P0442 is set, and condition (b) indicates a very small leak and DTC P0456 is set.