P04EF: EVAP System Very Small Leak Detected

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.

Low severity. Can I keep driving?Yes, you can normally keep driving. If there is a strong smell of gasoline, find the leak before continuing and do not smoke nearby.

Symptoms

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

Most likely causes

Purge valve not sealing
Vent valve or leak check module leaking
Hairline crack or leaking connections at the canister
Module software (misjudgement)

What to check, in order

Start with the simplest. You can tick off each step.

Start with the fuel cap: tighten it fully and check its seal
Inspect the hoses and connections around the tank and the charcoal canister
Use the scanner to test the purge and vent solenoid valves
Run a smoke test to find small leaks

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P04EF means

P: Powertrain: engine and transmission
0: Generic code: same on every make
4: Emissions
EF: Specific fault in that system

Frequently asked questions

Can I keep driving with code P04EF?

Yes, you can normally keep driving. If there is a strong smell of gasoline, find the leak before continuing and do not smoke nearby.

What causes code P04EF?

The most common causes are: purge valve not sealing; vent valve or leak check module leaking; hairline crack or leaking connections at the canister.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.

P04EF: EVAP System Very Small Leak Detected - Fresh Air Side