Fault codeP0451

MODERATE

Evaporative Emission Control System Pressure Sensor Range/Performance

What this code means

Evaporative Emission Control System Pressure Sensor Range/Performance.

P0451 is a generic code: it means the same thing on every make. P0451 belongs to family P04xx — Emission controls. Its page explains how to read a code and groups all its codes by part.

Type of fault

The signal stays within its electrical limits, but its value is not plausible: it does not match what the control unit expects given the other sensors. The sensor may be drifting with age, but it may also be faithfully measuring a real mechanical problem — a leak, a blockage, an air leak. Replacing the sensor without checking does not always clear the fault.

Fuel vapour recovery (EVAP)

The EVAP system traps fuel vapour from the tank in a charcoal canister, then burns it in the engine through a purge valve. A vent valve and a pressure sensor let the control unit test the system for leaks, often after the car has been parked.

What you notice

Usually nothing in driving; sometimes a fuel smell, a hard start just after refuelling, or a car that stalls at idle if the purge valve stays open.

How to check it

The fuel cap first: badly closed or with a worn seal, it is the most common cause of a leak code. Then the hoses around the tank and the canister, and the purge and vent valves (connector, resistance, valve that clicks when powered). Klaro shows the commanded purge and the EVAP pressure when the car reports them.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

Commanded evaporative purge
Evap system vapor pressure
Fuel level

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

How to read this code

PPowertrain
0standardised code
4Emission controls
51P0451