Powertrain, generic · Auxiliary emission controls

P04AE EVAP System Purge Control Valve B Performance/Stuck Open

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. P04AE is the function check of purge valve B: flow, tank pressure or mixture react although the valve is commanded closed. It is documented on Subaru's FA24 turbo engines (Legacy/Outback 2.4, WRX, Ascent) as a known false trigger, fixed by an engine module software update – the bulletin advises checking the software before replacing the valve. Subaru publishes no thresholds.

Likely causes

Engine module software (Subaru FA24: known false trigger)
Purge valve B leaking or stuck open (charcoal dust, wear)
Purge lines swapped after a repair
Tank pressure sensor misreading

Symptoms

  • Check engine light on
  • Rough idle or starting problems after refuelling
  • Often no change in driving (software false trigger)

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–350
Time0.5–1.5 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

P04AFP04ABP04ACP04ADP0496P0443

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