P0451

What it means

P0451 means the control unit has seen the signal from the evaporative emission (EVAP) system pressure sensor, or switch, drift out of its expected range or behave implausibly. It is usually set when the reading does not match what the system should show, for example a pressure that changes when it should not, sticks, or does not match the readings expected at key-on or with the purge and vent valves in a known state. The fault is in the sensor's signal or performance rather than being a simple open or short circuit.

Symptoms

• Malfunction indicator lamp on
• Fuel smell around the car, particularly after refuelling or when parked
• Difficulty refuelling, such as the pump clicking off repeatedly
• Failed emissions test or readiness monitors not completing
• Usually no change in driveability
• Occasionally an EVAP leak or related codes stored alongside

Possible causes

• Faulty or drifting EVAP pressure sensorlikely
• Blocked, kinked or damaged EVAP vent line or vent valve stuck closed, causing abnormal pressure or vacuum in the tanklikely
• Purge valve stuck open or partly open, creating unexpected vacuum in the systempossible
• Wiring or connector problems at the sensor, such as corrosion, chafing, high resistance or water ingresspossible
• Blocked or contaminated charcoal canister or restricted EVAP pipeworkpossible
• Faulty fuel tank, or fuel filler cap or neck problem affecting pressurerare
• Control unit fault or outdated software calibration, where a known issue is documented for the modelrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting fuel level, temperature and engine state when the code set
2. Check for related EVAP codes, as these help separate a sensor fault from a leak or valve fault
3. Inspect the fuel filler cap, seal and filler neck, and refit or replace the cap if it is damaged
4. Visually inspect the sensor, its connector, wiring and the EVAP pipes and hoses for damage, corrosion, kinks or water
5. View live data for the EVAP pressure sensor with the key on and engine off, and compare it with the expected value for the model, since the specified range varies
6. Command the purge and vent valves with a scan tool where supported, and watch the sensor reading respond as expected
7. Check the supply voltage, ground and signal wiring at the sensor against the manufacturer's data
8. Test the purge and vent valves and inspect the canister and vent path for blockage, following the model-specific procedure

How to fix

• Replace the EVAP pressure sensor if its signal is out of range or does not respond correctly
• Repair or replace damaged wiring, terminals or connectors at the sensor
• Clear a blocked or kinked vent line, or replace a vent valve that sticks
• Replace a purge valve that leaks or sticks open
• Replace a contaminated or blocked canister or damaged EVAP pipework
• Replace a faulty filler cap, or apply an updated control unit calibration only where the manufacturer lists it as a fix

Can I keep driving?

Nearby codes

• P0450GenericEVAP System Pressure Sensor/Switch A Circuit
• P0452GenericEVAP System Pressure Sensor/Switch A Circuit Low