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A pocket of air left in the cooling system after work was done on it, or drawn in through a leak.

It produces faults that look like sensor and valve failures, which is why it is worth ruling out first. A temperature sensor surrounded by air is measuring air, so it reads low, lags behind, or swings — and a module comparing that reading against what it commanded concludes the thermostat, the bypass valve or the pump is not doing its job. The parts are fine and the readings are not.

Modern engines trap air far more readily than older ones. There are several loops rather than one — heater matrix, turbocharger, EGR cooler, sometimes a battery or transmission circuit — with high points the coolant does not naturally push air out of, and many are specified to be filled under vacuum or bled through a documented procedure. A system topped up rather than bled is the ordinary way this arrives.

The recognisable signs are worth knowing. Gurgling from behind the dashboard, a heater that blows cold at idle and hot when revved, a temperature gauge that swings rather than settles, and a level that keeps dropping for the first few days after a repair as the pocket works its way out.

If the air keeps coming back, that is a different and more serious finding. Nothing draws air into a sealed system, so a circuit that will not stay full is being fed from somewhere — a leaking hose or seal, a pressure cap that no longer holds, or combustion gases entering through a head gasket.