False Coolant Temp Messages

[SaabC5]

I've recently started getting warning messages saying "engine coolant temperature too high" along with the big red STOP message on the dash, it lasts for a couple of seconds and then goes out, it then re-appears almost immediately and goes away again. I know its a false error because it always seems to happen when the car has been sitting overnight and the engine is stone cold. I've also noticed the coolant gauge starts to rise, will drop to zero and then suddenly spring up to the correct reading. I'm thinking the coolant temp sensor is knackered, would others concur or am i barking up the wrong tree?

[addo]

This was a problem with C3s. Wheeler will have more to add, but I understand it was due to fluctuating current draw in the cooling fan setup.

The fix was to quietly supersede the cooling fan unit for one that used a relay to switch heavy current.

[Peter.N.]

I frequently used to get this problem with XMs, providing the temperature readout was correct I ignored it and it would usually go away.

Peter

[wheeler]

9 times out of 10 just a faulty sensor, you can sometimes see a hairline crack in the metal part of the sensor when you remove it. Best to get it sorted quick as worst case coolant can leak into the sensor plug & capillary action can draw coolant up the wires & straight into the injection ECU. Never saw it happen on a C5 right enough but have seen it on a few C3's & Picasso's.
If I remember right the sensors are under £20 & take minutes to change.

[SaabC5]

Thanks Wheeler, new sensor ordered £13.50