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A pump's motor will not turn — the shaft is jammed rather than the circuit being faulty.

It is diagnostically one of the better faults, because it is a mechanical fact rather than a measurement. The module knows the difference: current is flowing and rotation is not happening, which no wiring problem produces. That rules out the whole electrical half of the usual list before anything is measured.

What jams a pump is nearly always what is in the fluid rather than the pump itself. Wear particles, sealant from a previous repair, or debris shed by a failing component upstream reach a pump whose clearances are measured in fractions of a millimetre. Bearings seize too, but they are the second answer rather than the first — and the distinction matters, because a pump replaced into contaminated fluid seizes again. Whatever is circulating has to come out with it.

One case is not a failure at all and is worth eliminating before anything is bought. Cold, thick fluid can stall a small pump on a hard frost, and the same pump runs perfectly once warm. So a stall reported on one freezing morning, with no repeat, is often the weather — and the wrong fluid specification makes that far likelier, because a fluid too viscous for the temperature range is a stall waiting for a cold night.