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.

 

[BlackC55]

It is very likely to be a faulty thermostat. Does your engine fan operate normally?

 

[Pete2222]

It is very likely to be a faulty thermostat. Does your engine fan operate normally?

I have not seen my engine fan working. I presume this comes on if the engine gets realy warm, which isn't happening. It has never bene running when I've been looking, with the engine idling.

 

[Oleks1992]

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.

If you actually use xentry to diagnose that fault code, it'll take you through a thermostat temperature build up test. So as an educated guess it could well be thermostat or flaps in the grill

 

[BlackC55]

I have not seen my engine fan working. I presume this comes on if the engine gets realy warm, which isn't happening. It has never bene running when I've been looking, with the engine idling.

Then its probably just a thermostat issue. As above; if you use some decent diagnostic software it will process the code for you.

 

[Supernoodle]

A previous post with some useful info on the cooling system. You'll see the OP there changed the stat too.

C180 1.6 M274 Coupe - Unstable Temperature | Engine

Hello, how are you? My name is Felipe, I'm Brazilian and I own a C180 1.6 2013/14 with the M274 engine. My car has an unstable temperature. In small circuits the temperature hangs in 90 degrees celsius But if I'm walking on a road at 120km/h the temperature drops to 65-70 depending. I saw...

forums.mbclub.co.uk

 

[markjay]

The thermostats on early M274 engines do fail.

It's a complex part (for a thermostat) with built-in electronics (and it actually has an electric heater to help warm up the coolant quicker). Plus, it's located under the engine, and getting to it requires a considerable amount of labour.

Around £700 parts and labour from a dealer, of roughly half that from an independent specialist.