Powertrain, generic · Auxiliary emission controls

P04EF EVAP System Very Small Leak Detected - Fresh Air Side

Check engine lightOn

EmissionsRelevant

Drive cycleRequired

Limp modeNo

Hybrids with a sealed fuel tank have a valve separating the tank from the charcoal canister and its vent side. CARB allows such sections to be leak-checked separately with their own codes; a Ford patent checks the tank side via tank pressure with the valve closed and the canister side with a leak check module. Conventional systems report the same leak as P0442, P0455 or P0456. P04EF reports a very small leak (0.020 in) in the canister and vent section; the leak check module compares the vacuum against a 0.020 in reference and the section does not reach it. On Ford's C-Max, Fusion and MKZ hybrids and Energi a leaking purge valve is the typical trigger, fixed per bulletin with the valve and software.

Likely causes

Purge valve not sealing

Vent valve or leak check module leaking

Hairline crack or leaking connections at the canister

Module software (misjudgement)

Symptoms

Check engine light on
EVAP readiness not set
No change in driving

Repair estimate

DifficultyMediumDIY possible

Parts and labour€40–350

Time0.5–1.5 h

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

Parts involved

charcoal canisterevap purge valveleak check moduleevap hoses

Related codes

P04EB P04ED P04EE P0456 P0496

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