Faulty coolant level sensor

The float or probe reporting how much coolant is in the expansion tank, failed or sticking. Most are a magnet in a float operating a reed switch in the tank wall, which is why they fail mechanically far more often than electrically — a float stuck by scale or by the gelled residue of an incorrect coolant does not move, and reports the level it was last at.

Reading falsely full is the failure that matters, because it removes the warning rather than raising a false one. An engine losing coolant slowly then reaches a dangerous level with nothing on the dashboard.

Its importance grows sharply on vehicles with more than one cooling circuit, and this is where it is most often overlooked. A hybrid or electric car has a separate low-conductivity circuit for its traction battery, and a turbocharged car often has a third for the charge air cooler — each with its own reservoir and its own sensor. A level warning on one of those is not about the engine at all, and topping up the wrong reservoir does nothing.

Before replacing it, establish whether the reading is honest. A level that genuinely is falling needs the leak found, not the sensor changed — and on a high-voltage battery circuit that is a safety matter rather than a maintenance one, because those systems use a specific coolant chosen not to conduct.