Fuel filler cap missing, loose, damaged or with a worn seal; Perished or damaged filler neck seal, or corroded filler neck; Cracked, disconnected or perished EVAP hoses and quick-release connectors on the tank side; Leaking or stuck-open vent valve or tank isolation valve (where fitted); Fault with the fuel tank pressure sensor or its wiring giving a false reading; Leaking fuel tank, tank seal, fuel pump or sender flange seal; Damaged fuel tank or lines from corrosion or road debris.

P04EC

EVAP System Large Leak Detected - Fuel Tank Side.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
4: AreaAuxiliary emission controls
EC: FaultThe specific fault in that area

What it means

P04EC means the control unit ran an evaporative emission leak test and found a large leak on the fuel tank side of the system, between the tank and the purge or isolation valve. The test normally pressurises or applies vacuum to the sealed system and watches the fuel tank pressure sensor. The pressure would not build or hold as expected, so the leak was judged large. The exact test method and the split between tank side and canister side depend on the model and engine.

Symptoms

Malfunction indicator lamp on
Fuel smell around the car or near the filler area
Fuel cap warning message on some models
Hissing from the tank or filler area after removing the cap
Difficulty refuelling or the pump nozzle clicking off early
Usually no change in how the engine runs

Possible causes

Fuel filler cap missing, loose, damaged or with a worn seallikely
Perished or damaged filler neck seal, or corroded filler necklikely
Cracked, disconnected or perished EVAP hoses and quick-release connectors on the tank sidelikely
Leaking or stuck-open vent valve or tank isolation valve (where fitted)possible
Fault with the fuel tank pressure sensor or its wiring giving a false readingpossible
Leaking fuel tank, tank seal, fuel pump or sender flange sealpossible
Damaged fuel tank or lines from corrosion or road debrisrare

How to diagnose

Read the stored codes and freeze frame data, and note whether other EVAP codes are present, such as purge valve or pressure sensor faults.
Check that the fuel cap is fitted, tightens or clicks correctly, and that its seal is intact and clean. Check the filler neck sealing surface.
Visually inspect the filler neck, tank-side EVAP pipes, hoses and quick-release connectors for cracks, disconnection or corrosion.
Inspect the wiring and connectors of the tank pressure sensor and the vent or isolation valve for damage, corrosion or water ingress.
Use live data to check the tank pressure sensor reading against ambient pressure with the system open, and check that it responds sensibly.
Operate the vent and isolation valves with a bi-directional scan function and check that they open and seal as commanded.
Smoke test the EVAP system at low pressure, following the manufacturer's procedure for the model, to find the leak. Check the tank, pump flange seal and filler area.
Check for fuel smell or wet areas around the tank and fuel pump access area, as a leaking tank is a fuel leak and a safety concern.

How to fix

Replace the fuel cap with a correct one if it is damaged, worn or not sealing.
Repair or replace the filler neck, its seal or damaged tank-side EVAP pipes and connectors.
Replace a leaking or sticking vent valve or tank isolation valve.
Repair wiring faults or replace a faulty tank pressure sensor, as confirmed by testing.
Replace the fuel pump flange seal or the fuel tank if it is leaking or corroded.
After the repair, clear the code and complete a drive cycle so the EVAP monitor runs and confirms the fix.

Can I keep driving?

Low severity

The car can normally be driven while you book it in, as this fault does not usually affect engine running. If you smell fuel strongly or can see fuel leaking, stop driving and have it checked straight away.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.