Coolant leak into the sensor connector

Coolant has found its way into an electrical connector, usually from a weeping joint, seal or porous housing directly above or beside it.

It produces a genuine electrical fault with undamaged wiring, which is what makes it worth naming separately. Coolant conducts, so it bridges terminals and gives readings that are electrically real and physically meaningless — a circuit reading low that measures perfectly once the plug is dried. And it is corrosive to the plating on terminals, so what begins as a bridged connection becomes an eaten one: an intermittent fault first, a permanent open eventually.

The consequence for repair is the important part: replacing the component does not fix it. A new sensor or valve plugged into a contaminated connector is sitting in the same coolant, and it fails the same way, usually within months. Finding where the coolant is coming from is the repair, and the part named by the code may well be perfectly good.

The tell is visible once somebody unlatches the plug — green, white or crusty deposits on the terminals, or damp inside the housing. It is worth asking for explicitly, because a connector is normally checked for being connected rather than for being clean, and nothing about the outside of it looks wrong.

It concentrates around thermostat housings, coolant valves and temperature sensors, where connectors sit in the flow of hot coolant surrounded by seals and hose joints — and on engines with plastic housings that become porous with age, the housing itself is often both the source and the eventual repair.