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

What does P0451 mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

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

Common causes

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

Likelihood is a general estimate across vehicles; your make and model may differ.