Diagnostic Aids

WARNING: This page is about a different car, the 2003 Lexus LX 470. However, it is still accessible from the selected car via links, so may be relevant.

The vapor pressure sensor, the VSV for the Canister Closed Valve (CCV) and the VSV for the pressure switching valve are used to detect abnormalities in the evaporative emission control system. The ECM decides whether there is an abnormality in the evaporative emission control system based on the vapor pressure sensor signal. DTC P0442 or P0456 is recorded by the ECM when evaporative emissions leak within charcoal canister, or when the vapor pressure sensor malfunctions.

DTC is detected after cold engine start. After VSV for EVAP operation, the VSV for EVAP is turned off, sealing the vacuum in the system and the ECM begins to monitor the pressure increase. Some increase is normal. A very rapid, sharp increase in pressure indicates a leak in the EVAP system and sets the DTC P0442. This monitoring method is also able to distinguish what is called the small leak detection for DTC P0456. A pressure rise just above normal indicates a very small hole. Possible causes are:

Hose or tube cracked, damaged or loose seal.
Fuel tank cap incorrectly installed.
Fuel tank cap cracked or damaged.
Vacuum hose cracked, blocked, damaged or disconnected.
Fuel tank cracked or damaged.
Charcoal canister cracked or damaged.
Open or short in vapor pressure sensor circuit.
Faulty vapor pressure sensor.
Fuel tank overfill check valve cracked or damaged.
Faulty ECM.