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A thermostat jammed shut, so coolant cannot circulate to the radiator. It is the failure mode that overheats an engine quickly, and it does so with a full cooling system, a working pump and clear radiator — which is why the usual checks all pass.

The signature is specific and worth knowing: the top hose gets hot while the bottom hose stays cold. Coolant is circulating within the engine and not reaching the radiator, so the difference across the radiator is the whole diagnosis and takes seconds to feel. Temperature climbs steadily regardless of speed, unlike an airflow problem which eases at speed.

It is genuinely urgent. An engine run with a closed thermostat reaches damaging temperatures fast, and the sequence ends in a head gasket or worse. Do not diagnose it by continuing to drive. Overheating also produces knock codes and sensor faults as consequences, so a cluster of unrelated codes on an engine that has run hot should be read as one event.