P0451: EVAP System Pressure Sensor/Switch A Circuit Range/Performance

The evaporative emission (EVAP) system stores fuel vapour from the tank in a charcoal canister; a purge valve lets the engine draw it off, and a vent valve opens the canister to fresh air. The tank pressure sensor measures small pressures around ambient for the leak tests. Its direction differs by maker – on GM vacuum raises the voltage, on Ford and Bosch pressure does – so low and high refer to the signal voltage only. P0451 is its plausibility check. GM checks during the engine-off leak test that the signal settles to ambient pressure with the vent open and averages several tests; Ford checks the offset after at least 4–6 hours off (±0.5 kPa). Saab's Bosch ME9 watches start pressure, the drop during purging and excessive oscillation. On Subaru a leaking or blocked hose between sensor and tank is a frequent cause.

Low severity. Can I keep driving?Yes, you can normally keep driving. If there is a strong smell of gasoline, find the leak before continuing and do not smoke nearby.

Symptoms

Check engine light on
EVAP readiness not set, emission inspection not passed
No change in driving

Most likely causes

Tank pressure sensor offset shifted or sluggish
Vent line or canister blocked (pressure does not release)
Sensor hose or port leaking, kinked or blocked
Tank overfilled, fuel in the lines

What to check, in order

Start with the simplest. You can tick off each step.

Start with the fuel cap: tighten it fully and check its seal
Compare the reading with other engine data while varying the engine speed.
Inspect the hoses and connections around the tank and the charcoal canister
Use the scanner to test the purge and vent solenoid valves
Run a smoke test to find small leaks

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P0451 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
4: Emissions
51: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0451?

Yes, you can normally keep driving. If there is a strong smell of gasoline, find the leak before continuing and do not smoke nearby.

What causes code P0451?

The most common causes are: tank pressure sensor offset shifted or sluggish; vent line or canister blocked (pressure does not release); sensor hose or port leaking, kinked or blocked.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.