Sensor o interruptor del tapón de combustible defectuoso

The switch or sensor that tells the module whether the filler cap is fitted and sealed, so the evaporative system can distinguish a leak it should report from a cap somebody left off. Its entire purpose is to stop the system blaming the vehicle for the driver's action, which is also why a fault here is more irritating than serious.

A failure inverts that logic in one of two ways. Reporting the cap absent when it is fitted suppresses or misattributes genuine leak testing, so a real evaporative fault can hide behind it. Reporting it present when it is off means the system runs a leak test into open atmosphere and stores a large leak that has an obvious explanation nobody was told about. The second is the commoner complaint and the first is the more damaging.

Check the mechanism before the electronics. These switches sit at the filler where they meet water, road salt and the flap being slammed, so a sticking plunger, a corroded contact or a bent bracket accounts for most of them. Operate the cap while watching live data — a switch that does not change state on a cap that plainly seals is the whole diagnosis.