What does it mean?
The module has decided that the engine thermostat is not regulating coolant temperature the way its model expects. The software builds a picture of how quickly coolant should warm up and where the temperature should settle, using engine load, running time, ambient temperature and vehicle speed, then compares this with the actual coolant temperature.
If the engine warms far too slowly, never reaches its target, or the temperature wanders instead of holding steady, the thermostat is judged to be performing poorly. On engines with an electrically heated map-controlled thermostat the module also checks whether its own commands produce the expected change in temperature. Running too cold keeps the engine in a rich warm-up strategy for longer, which costs fuel and raises emissions, so the fault is more than a comfort issue.
Symptoms
- Engine takes much longer than usual to warm up and the gauge sits lower than normal
- Weak cabin heat in cold weather, and heat that fades at highway speed
- Higher fuel consumption, especially on short trips
- MIL on with a freeze frame usually taken during a cold start
- Cooling fan may run more than expected on some vehicles
- Occasional overheating instead, if the same thermostat sticks closed intermittently
Likely causes
- Thermostat stuck open or opening well before its rated temperature
- Wrong temperature rating fitted during a previous repair, or the thermostat left out altogether
- Heater element circuit fault on a map-controlled thermostat, so the module cannot influence opening
- Coolant temperature sensor reading inaccurately, which distorts the comparison
- Low coolant level or air trapped in the system after a service
- Cooling fan running constantly because of a control fault, keeping the engine below its target
- Thermostat sealing face damaged or debris holding the valve off its seat
- Very cold ambient conditions combined with a marginal thermostat, which exposes a borderline unit
How to fix it
- Verify the coolant temperature sensor first by comparing live data with an infrared reading of the thermostat housing and with intake temperature after an overnight soak
- Log a full warm-up from cold and note how long the engine takes to reach normal temperature and whether it holds there
- Feel or measure the upper and lower hose temperatures during warm-up; a lower hose that gets hot too early points at a thermostat that is already open
- On a map-controlled thermostat, check the heater element resistance and command it with a scan tool while watching the temperature respond
- Check coolant level and bleed the system properly, since air pockets distort the readings
- Confirm the cooling fan is not being commanded on continuously
- Fit the correct thermostat for that engine, refill and bleed according to procedure, then repeat the warm-up log to confirm normal behaviour