Cause
Faulty brake pad wear sensor
The wear sensor in a brake pad, faulty or its circuit damaged. The sensor is a loop of wire moulded into the pad that the disc cuts through as the friction material wears — so it works by being destroyed, and it reports wear by going open circuit.
That makes a broken wire and a worn pad electrically identical, which is the whole difficulty. Check the pads before the sensor: if they have material left, the fault is the harness. The lead runs to the caliper, flexing with the suspension and exposed to road salt, so a chafed wire or a corroded connector is common. Sensors are usually single-use and must be replaced with the pads; refitting an old one that has already been cut reports worn pads immediately on a new set.
Typically specialist work. Needs equipment or training most home workshops do not have.
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