Powertrain, generic · Auxiliary emission controls

P04AF EVAP System Purge Control Valve B Stuck Closed

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. Purge valve B is a second electric purge valve, usually for the high-load purge line of a turbo engine, which purges to the compressor inlet when there is boost in the manifold (Ford patent). Many makers solve this path with one purge valve and passive check valves instead – Ford tests it as P04F0 – and then have no valve B. CARB requires every purge path to be checked. P04AF is the flow check of the high-load path: under boost the module opens purge valve B, and if tank pressure or mixture do not react, the valve is stuck closed or the path is blocked – a FCA patent describes such a stepwise test. The canister is then not purged under boost and can saturate on load-heavy driving. No manufacturer document available to us uses this code; the description follows CARB and the comparable purge valve A checks (for GM P0443/P0458/P0459).

Likely causes

Purge valve B stuck closed (gummed, dirt)
Line to the compressor inlet off, kinked or blocked, check valve sticking
Engine module software or false trigger
Canister or vent blocked

Symptoms

  • Check engine light on
  • Fuel smell
  • EVAP readiness not set

Repair estimate

DifficultyMediumDIY possible
Parts and labour€60–300
Time0.5–2 h

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

Parts involved

evap purge valve bhigh load purge linepurge valve wiringengine control module

Related codes

P04AEP04ADP04ABP0497P04F0

References: law.cornell.edu, patents.google.com, patents.google.com, static.nhtsa.gov.