Cause
Loose or missing fuel filler cap
The filler cap left loose, cross-threaded or off altogether. It is the first thing to check on any evaporative leak code and the commonest single cause of one — the cap seals the tank, and the system tests the tank as a sealed vessel.
The standard names it directly, which is unusual: one code exists specifically to say the cap is loose or off, precisely because the fault is so common and so cheap. A cap that has not been turned until it clicks is enough, and so is one cross-threaded after refuelling.
Two things separate it from a cap that is genuinely faulty. A loose cap seals perfectly once tightened, so the code clears and stays clear; a perished seal or a non-original cap of the wrong pressure rating produces the same leak with the cap correctly fitted. Check the seal's condition while the cap is off — soft and uniform is serviceable, hard or flattened is not. The monitor only runs under particular conditions, so a code can persist for a drive cycle or two after the cap is tightened, which is worth telling an owner before they buy a cap they do not need.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (10)
Codes where this is a likely explanation first, rather than merely a possible one.
- P0440 Evaporative Emission System Malfunction Likely for this code Low
- P0442 Evaporative Emission System Leak Detected — Small Leak Likely for this code Low
- P0455 Evaporative Emission System Leak Detected — Large Leak Likely for this code Low
- P0456 Evaporative Emission System Leak Detected — Very Small Leak Likely for this code Low
- P0457 Evaporative Emission System Leak Detected — Fuel Cap Loose or Off Likely for this code Low
- P04EC Evaporative Emission System Large Leak Detected — Fuel Tank Side Likely for this code Low
- P2404 Evaporative Emission System Leak Detection Pump Sense Circuit Range/Performance Likely for this code Moderate
- P2409 Fuel Cap Sensor Switch Circuit Range/Performance Likely for this code Low
- P2408 Fuel Cap Sensor Switch Circuit Low
- P2412 Fuel Cap Sensor/Switch Circuit Intermittent/Erratic Low
Information, not instruction
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