Powertrain, generic · Auxiliary emission controls

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

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

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

Symptoms

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

Repair estimate

DifficultyMediumDIY possible
Parts and labour€40–300
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

fuel tank pressure sensorsensor wiringengine control module

Related codes

P0450P0452P0453P0454P0442P0446

References: gsi.ext.gm.com, fordservicecontent.com, saabwisonline.com, saabwisonline.com.