Faulty electric coolant pump

An electrically driven coolant pump has failed — the motor, its impeller, or the electronics driving it.

These pumps exist because a belt-driven one is wrong about when cooling is needed. A belt pump turns in proportion to engine speed, so it moves least in slow traffic where an engine is hottest and most at a motorway cruise where it is not. An electric pump is commanded, so it runs when there is heat to move and stops when there is not. On an electrified vehicle that argument becomes non-negotiable rather than merely better, because the things needing cooling — the inverter, the charger, the battery — do their hardest work when the engine is off or absent entirely.

Which is what makes their failures easy to miss. The pump that matters may be one the driver has never heard of, serving a circuit that is not the engine's, running at times the car appears to be doing nothing at all. So a failed electronics or battery pump often shows up as charging that has quietly become slow, because the car protects hardware it cannot cool by asking less of it — and nobody connects a longer charge to a coolant pump.

Two things are worth eliminating before the pump is condemned, and both are cheaper. Is it being fed? A pump correctly commanded with no supply is indistinguishable from a failed one, and a fuse, relay or connector costs a fraction of the part. And is there coolant to move? These circuits are separate from the engine's, with their own reservoir that routine servicing often ignores, and they are awkward to bleed — so air trapped after a repair stops flow while the pump runs perfectly. A pump replaced to cure an airlock fails to fix anything and the airlock is still there.

Where the pump genuinely has failed, the usual modes are a seized or worn bearing, an impeller no longer gripping its shaft, or integrated electronics that have given up — and on many designs those electronics are not separately available, so the whole assembly is the part.