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The metal contacts inside a switch have corroded, so it no longer makes a clean connection when operated.

A switch is a deceptively hostile place for a metal contact. Every operation scrapes the surfaces together, which is what normally keeps them clean — and a switch that is rarely used loses that self-cleaning entirely. So the switches that corrode are often the ones nobody touches, which is why a fault appears on the day something is finally needed.

Where the switch lives decides how likely this is, and the exposed ones are a category of their own: boot releases, charge port buttons, tailgate switches and door handles are pressed with wet hands, washed at pressure, and salted all winter. Anything on the outside of the car should be suspected early.

The behaviour is characteristic and worth recognising. A corroded switch rarely fails outright — it becomes unreliable, working most of the time and failing intermittently, more often in the wet or the cold. That intermittency is what gets it dismissed as a glitch, and it is exactly the signature.

It is also one of the few faults with a genuine home remedy. Many switches respond to being operated firmly a few dozen times, which abrades the film away; contact cleaner does the rest where the switch can be reached. What does not work is a general-purpose lubricant — it attracts grit and leaves a residue that makes the contact worse within months. Where a vehicle offers another route to the same function, trying it is also a free diagnosis: if the touchscreen or the app works and the button does not, the switch is confirmed.