My 2015 1.0 125 has always kept a pretty constant engine temperature, once the engine has warmed up, which is dead on the halfway mark on the gauge. However, I noticed on a fairly long journey the other day, that the temperature on the gauge was slowly dropping down to the quarter mark, then slowly rising again so I stopped and checked the coolant level, which was was okay. The only thing I'd had done recently was have the air-con re-gassed so I turned it off and the coolant temperature went back to normal, with the gauge sitting on the halfway mark. I've never noticed this happen previously with the air-con on so could the thermostat be knackered, as the radiator fan seems to be cutting in and out as normal? Cheers.

That would be where I'd start TBH. To counter the main fan running all the time when the AC is on (by design), the thermostat shoud close if/when the coolant temp gets below whatever the trigger temp is for the thermostat.

 

As you mentioned yourself, it sounds like a classic case of the thermostat sticking open, so hopefully it's an easy/simple fix.

2 hours ago, FatHead1979 said:

That would be where I'd start TBH. The counter the main fan running all the time when the AC is on (by design) the thermostat shoud close if/when the coolant temp gets below whatever the trigger temp is for the thermostat.

 

As you mentioned yourself, it sounds like a classic case of the thermostat sticking open, so hopefully it's an easy/simple fix.

Thanks for the reply. I think it is the thermostat, as it's also taking much longer to reach the normal temperature.

Hi, I had exactly the same albeit on my Mk3 Mondeo. Temp gauge was always solid in the middle for all the years I had the car.  Then started fluctuating, down to a quarter then up to three quarters.  All over the place. I suspected sender unit so replaced it.  No difference.  Replaced thermostat. All back to normal now.