Wasserpumpe defekt

The pump circulating coolant through the engine, and the fault worth understanding here is not that it stops but that it can keep turning while moving almost nothing — which is what makes it so often missed. A seized pump announces itself immediately; a pump with an eroded or detached impeller lets the engine warm up, idle and behave normally, and only fails to cope when heat load rises.

That produces a distinctive pattern. Temperature is normal in traffic and climbs on a sustained climb or at motorway speed — the reverse of the airflow-related overheating everybody expects — because the fault is circulation rather than heat rejection. Cooling fans running hard against a hot engine while the top hose stays cool is flow, not airflow.

The consequences reach beyond temperature. Poor circulation makes coolant temperature readings inconsistent between sensors in different places, and localised hot spots promote detonation that a knock sensor genuinely hears — which is why this cause appears on knock and sensor-correlation codes rather than only on over-temperature ones. Check for flow directly, and treat two coolant sensors that disagree as a circulation question rather than assuming one of them has drifted.