I have just had my 2013 Mk3 Focus serviced by my garage and on collection I found a fault that was not present when I took the car in!

The external temperature monitor is reading -40C, and the red ice warning light is on.  I immediately drove back to the garage as my initial thought was that a wire had probably been knocked off the sensor during the servicing.  The garage ascertained that the sensor is an integral part of the nearside door mirror and in the absence of any other information have ordered a replacement, assuming that the unit had been damaged somehow while in their care.

I was surprised to see on my drive home that the temperature reading is gradually increasing towards zero - which I would definitely NOT expect if it was broken.  Apparently the ODB analyser at the garage can see no fault, so the car is booked back in for the mirror to be replaced.

Does anyone know anything about possible causes?  I am not going to cancel the booking yet - but I THINK the sensor reading MAY just be slowly coming back to correct.  Could the OBD unit at the garage perhaps have wiped some reading, causing the warning to be issued? I am assuming their kit is NOT specific to Ford - but a general high grade OBD analysis tool.   I know that some readings on the engine control unit are lost if power is disconnected and that they are then gradually re-learned as the car is driven. Is this possibly the same for this warning system?   If so, I am very surprised it has not occurred before, as I have used the garage before (NOT a Ford dealership) for my servicing as I believe they are enirely competent and much cheaper than the Ford dealership.

I am keeping my fingers crossed that the readings WILL gradually come back to normal - but if not, the Plan B, is for the garage to continue with their plan to replace the unit.

This is totally normal on the Mk3s...no one knows why!  It will come back over a couple of days.