False overheating

Citroen C3 1.4 Desire on an 06 plate.

Getting false overheating warnings.

This occurs when the engine has been under load, such as going up hill or travelling at 70 mph, then allowing the car to continue to travel under its own momentum with foot off the accelerator whilst going down a lengthy incline.

Stopping the car and opening the bonnet the radiator fan is running but there is no typical overheating smell, the expansion tank can be opened with out the coolant boiling out or steam, the top hose and bottom hose on the radiator are the same temperature (both are moderately warm, not hot at all).

Disconnecting the ecu plug nearest the rear of the engine compartment causes the fan to stop and the overheat warning to disappear. The fault will reoccur if the above mentioned conditions of driving happen again.

I have replaced the one sensor I can find and this seemed to fix things but, this was temporary as the fault is now back. Disconnecting and reconnecting the plug on the sensor seems to make the fault disappear much like disconnecting the ecu plug but for longer.

I have noticed that the heater does not seem to get as hot as you would expect and combining this with the top and bottom radiator hoses being the same temperature (both are moderately warm, not hot at all), leads me to suspect that the thermostat is not closing. How this then leads to an overheat warning I don't understand.

The car also continuously gives me the "blip battery spent" warning . This can be cleared but returns once the car has been locked and unlocked with the remote.

Many thanks for any suggestions on how to proceed and possibly effect a cure

There are two temperature sensors. One green one on the top of the thermostat housing. This is used to derive the indicator on the instrument panel. The other is blue and is on the block, just under the thermostat housing. You need to remove the air cleaner box to get access to it. This one is used by the engine ECU for engine management. Both go through the engine ECU. Software engineers do crazy stuff if they don't get enough resources! For example there is no "your engine is running too cold" light and the Check Engine Light is reserved for when really bad stuff happens. I dunno...just guessing.

Byron wrote: ↑Thu Jul 18, 2019 2:14 pm the expansion tank can be opened with out the coolant boiling out or steam, the top hose and bottom hose on the radiator are the same temperature (both are moderately warm, not hot at all).

I have noticed that the heater does not seem to get as hot as you would expect and combining this with the top and bottom radiator hoses being the same temperature (both are moderately warm, not hot at all), leads me to suspect that the thermostat is not closing. How this then leads to an overheat warning I don't understand.

I would forget about the temp light coming on at the moment and check to see if your thermostat is OK. If your coolant system is operating properly it shouldn't get any cooler than about 90 degrees Celsius (when running) and that's hot to touch.