Cause
Perished fuel cap seal
The rubber seal on the fuel cap, hardened or cracked with age so it no longer seals the tank. It is distinct from a cap left loose — the cap is correctly fitted and clicks home, so every visual and tactile check passes, and the driver is certain the cap is on because it is.
The evaporative system tests the tank as a sealed vessel, so a seal that has lost its compliance leaks slowly and stores a small-leak code. Small-leak codes are the ones this fault produces, and they are intermittent by nature because the leak rate varies with temperature, fuel level and how recently the cap was turned.
It is the cheapest repair in the evaporative system and the first thing to eliminate. Look at the seal rather than at the cap: a serviceable one is soft, uniform and springs back; a failed one is hard, shiny, flattened or split. A non-original cap is worth suspecting too — these seals are specified to hold a pressure and vacuum, and a generic replacement often will not, which produces the identical code with a brand new cap fitted.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (4)
Codes where this is a likely explanation first, rather than merely a possible one.
- 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
- P04EE Evaporative Emission System Very Small Leak Detected — Fuel Tank Side Likely for this code Low
- P2409 Fuel Cap Sensor Switch Circuit Range/Performance Likely for this code Low
Information, not instruction
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