Ok ran into this the past couple days. I have a 2007 MCS. I have an OBD2 reader installed so I can watch all my temps and levels. I drive to work outside temp around 60f. Use the heat set 76f inside the car no issues on a 17 mile ride to work. Car coolant temp is 219-223f the whole way to work. On the way home the outside temp is 72f. So I start driving and turn on the a/c to max. Watch the coolant temp go from 223f down to 172f and I get 2 codes. P0128 and 2f07. Cel does not come on, but I can see the codes. Anyone else run into this?

 

The coolant thermostat may be bad.

https://bmwfault.codes/

 

I have no problem changing the thermostat, but I don’t want to throw parts at it. Drove it this morning and it was around 50 degrees. Had the cabin heat set to 78. The temp got to 221-225 degrees and never got a code.

 

The code and symptoms fit with a faulty thermostat.

Other items to check:

Is the coolant level in the reservoir low?

Is air trapped in the cooling system? Did you try bleeding the cooling system?

 

Coolant level hasn’t moved. It’s at the seam of the bottle. It’s always been there. Never had the need to bleed the system, but I can give it try.

 

https://www.northamericanmotoring.co...mini-code.html

 

I went through and read that post. Thing is, is my car gets to temp fine. 221-225f. It’s when I turn the max A/C on that I watch the temp dip all the way down to 170f and then I see the codes pop up. I drove it home today it was 80f out. Had nothing running, no a/c. No code popped up and it fluctuated between 219f and 225f. Drove like dream. Pulled into my drive way. Popped the hood while it was running at 225f, turned on the max a/c. Heard the fan kick on low for a minute. It then kicked to high for a minute, then back to low. Watched the temperature drop to 180f. Not sure if driving it would’ve dropped it lower, but that seems to be the case. No error popped up as it sat idling. I have no coolant loss or drips anywhere in the engine bay or on the ground. Dumbfounded.

 

Your observations still seem consistent with a faulty thermostat. Here's my interpretation:

Turning on the A/C causes the radiator/condenser fan to run, which in turn lowers the coolant temperature in the radiator. The drop in coolant temperature is detected by the coolant temperature sensor, which in turn sends the information to the DME, which is programmed to command the thermostat to close. The failure of the thermostat to close and to raise the coolant temperature results in the unexpectedly low coolant temperature reading and thereby code 2F07.

 

I’m thinking you’re correct. I’ll be ordering a new thermostat and water pipe. Do you suggest anything in particular that might not break the bank?

 

Start at the ECS Tuning site
Are any of the thermostat housing kits in your budget?
Maybe other members will have additional suggestions.