For 1990-2009 cars

### Circuit Description

The evaporative emission (EVAP) system is used to store fuel vapors as to reduce the amount of unburned fuel from escaping into the atmosphere. The function of the fuel tank pressure (FTP) sensor is solely for EVAP leak diagnostics. The fuel tank pressure sensor is a transducer that varies resistance according to changes in fuel tank pressure, the difference between fuel tank and outside air pressure. The ECM measures the signal voltage from the fuel tank pressure sensor, which is a reduced value of the ECM 5-volt reference. The signal voltage will read low when the pressure in the fuel tank is low (vacuum) and will increase as the pressure in the tank increases. DTC P0451 will set if the fuel tank pressure sensor signal voltage at the ECM is above a calibrated voltage value considered unrational.

### Conditions for Setting the DTC

DTC P0451 will set if the fuel tank pressure sensor signal voltage is greater than 4.5 volts, 18.75 mm Hg (10.04 in H2O) when:

• | The condition exists for longer than 3 seconds.

• | The startup ECT is less than 33°C (91°F).

• | The engine speed is at idle.

• | The engine run time is greater than 10 seconds.

DTC P0451 diagnostic runs once per ignition cycle once the above conditions have been met.

DTC P0451 is a type B DTC.

### Diagnostic Aids

Important: If DTC P0453 is also set, diagnose that DTC first.

FUEL TANK PRESSURE SENSOR on the scan tool should read between 2.35-2.85 volts with the ignition ON and the engine OFF and the fuel cap removed. This represents atmospheric pressure or 0 inches of vacuum.

DTC P0451 indicates that the fuel tank pressure sensor signal voltage is skewed high. Possible causes of DTC P0451:

• | Corrosion in the fuel tank inline harness connector

• | Corrosion in the engine I/P inline harness connector

• | Corrosion in the fuel tank pressure sensor harness connector

| Important: Inaccurate readings will occur if resistance measurements are taken on a fuel tank pressure sensor. The fuel tank pressure sensor contains an internal amplifier circuit that requires applied voltage to function properly.

• | Fuel tank pressure sensor skewed