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Coolant is not moving through part of the circuit as freely as it should — a blocked passage, a collapsed hose, a clogged heat exchanger, or air where liquid should be.

The reason this cause is worth separating from a failed pump is that everything appears to be working. The pump turns at the speed commanded, its circuit tests perfectly, the level looks right, and the heat still is not going anywhere. Nothing electrical will find it, which is why it tends to arrive after a component has already been replaced.

Air is the commonest single cause and the one most worth checking first. Modern cooling circuits — and especially the separate loops serving a battery, an inverter or a charger — run through components mounted at awkward heights and are genuinely difficult to bleed. A pocket of air in the wrong place stops flow completely while the pump spins freely. So any coolant work in the recent past moves this to the top of the list, and the fix is a proper bleed rather than a part.

The physical restrictions are the ordinary ones: deposits and corrosion products narrowing passages, a hose collapsing under suction rather than leaking, debris packed into a heat exchanger's fins, or the residue left by a stop-leak additive — which is worth asking about directly, because it blocks exactly the narrow passages it was poured in to seal.

The diagnostic that costs nothing is temperature. A circuit with good flow shows a clear difference between what goes in and what comes out; a circuit with almost no difference is not moving coolant, whatever the pump is doing. Feeling along a hose run or across a cooler for cold patches finds a blockage in minutes.