Low coolant temperature C200 W205 M274 just when A/C cabin fan is on code P0128 . Advice please.

[Pete2222]

The coolant temperature gauge on the dash quickly reached and stayed at 90C fine as it should when I first got the car.
I later noticed this started dropping intermittently and the orange dash light came on. Code its P0128 (coolant temp below range).
After some months I've learnt I can keep this coolant temperature completely normal (90C) provided I don't switch the A/C or cabin fan on. As soon as I switch the cabin fan on the coolant temperature massively drops. In 10 sec it goes for 90C down to 40C or even off the bottom of the display when freezing outside as it was recently.
Can anyone tell me if this is a faulty thermostat (which I've been quoted £500 for and it seems there are none in stock) or some other issue? The car seems to run fine, fuel consumption is fine. I have taken the temperature of the pipe that goes from radiator to engine a few times (using an IR thermometer). Its usually about 50-55C there if that helps.
Thanks for your help.

 

[Joshi]

I have exactly the same problem on my C200 W205 Petrol 2017 model.
Did you get to the bottom of this please ?

 

[MVD]

I have exactly the same problem on my C200 W205 Petrol 2017 model.
Did you get to the bottom of this please ?

Hello,

I thought i had seen this before. Check the screenshot it shows the TIPS document with the remedy:

 

[mioba]

The stat on those is within a housing with electrics and heating elements.
Not a classic wax stat you unbolt and chuck in a new one.
Furthermore they are not easy to get at require some labour.

500ish seems the rate. Find a good merc to do the job.

Definitely the stat.

 

[Joshi]

Thank you, but what is a stat ?

 

[Joshi]

Also, when I turn heater on, the temperature goes down very quickly. Is it ok ot drive around with heater on ? I am concerned the Engine Managment Computer thinks the temperature is low and so with holds coolant...when really the temperature is too high - resulting in engine damage.

 

[mioba]

thermoSTAT

Fix the issue. it maintains engine temp.

Dont overthink it