Cause
Failed thermostat heater element
The electric heating element inside a map-controlled thermostat, failed so the module can no longer force the thermostat open early. These thermostats open on coolant temperature like any other, but carry a heater that lets the module open them sooner when load or ambient conditions call for it.
So the failure does not stop the engine cooling — it removes the control. The thermostat still opens at its mechanical temperature, and the engine runs slightly hotter than the map intends under high load. The complaint is usually nothing at all, with only a code, which is why this fault is often first seen at a service.
The element is a resistive heater and fails open, so the circuit tests as open at the connector and the module reports it. Check the harness before the thermostat, since replacing one means draining the cooling system. A thermostat stuck open is a separate and more noticeable fault — an engine that never reaches temperature, poor heating and a rich warm-up — and the two are frequently confused because both name the thermostat.
Typically a moderate job.
Codes this can cause (3)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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