What does it mean?
This is not a sensor circuit code but a report that the engine actually ran hotter than it is allowed to. The control module compares the coolant temperature signal, and on many engines a modelled cylinder head temperature as well, against an overheat threshold and stores this fault when that limit is exceeded for a defined time. Depending on the vehicle it will also reduce power, limit boost, keep the cooling fans at full speed and in severe cases cut cylinders to protect the engine. The code tells you that heat was really present, so the diagnosis belongs to the cooling system rather than to the electronics. Treating it as a false alarm is dangerous, because sustained overheating damages head gaskets, heads and bearings very quickly.
Symptoms
- Temperature gauge climbing into the red or a high coolant temperature warning on the display
- Steam, a sweet coolant smell or visible coolant loss around the front of the engine
- Cabin heat that suddenly goes cold, which usually means the coolant level has dropped below the heater core
- Noticeable power reduction, limp mode or the engine refusing to rev
- Coolant boiling or overflowing from the expansion tank after shutdown
- Knocking or run-on after a hot shutdown in severe cases
Likely causes
- Low coolant level from an external leak at a hose, radiator, water pump seal or heater core
- Thermostat stuck closed or opening far too late
- Cooling fan not running because of a failed fan motor, relay, control module or a seized viscous clutch
- Water pump failure, including an eroded or slipping impeller that looks fine from outside
- Radiator blocked externally by debris or internally by scale and old coolant
- Combustion gases entering the cooling system through a failing head gasket, which pushes coolant out and creates air pockets
- Air trapped in the system after a repair because it was not bled properly
- Collapsed lower radiator hose or a stuck pressure cap that lowers the boiling point
How to fix it
- Do not open the system while it is hot; let it cool, then check the coolant level and look for external leaks
- Compare the coolant temperature in live data with an infrared reading of the thermostat housing to confirm the sensor is telling the truth
- Pressure-test the cooling system and the cap to find leaks that only show under pressure
- Command the cooling fans on with a scan tool and confirm they run at both speeds and pull air properly
- Check the thermostat by watching how the upper and lower hose temperatures change during warm-up
- Test for exhaust gases in the coolant, especially if the expansion tank is pressurising or bubbling
- Inspect the radiator core for blockage and check the water pump for play, leakage or impeller damage
- Refill and bleed the system exactly as the manufacturer specifies, then road test while logging temperature