[estrellajon]

So with an error of P0128, I did a full cooling system overhaul including new water pump, coolant switch, exp tank and new coolant. After say a week, the error came back on. I used a Wahler thermostat

Any ideas

[PhaseP]

One possibility you forgot to connect the wire for thermostat heating element on the thermostat. I would check that first since you just put a new thermostat my understanding from what you wrote.

If you didn't forget to connect the wire then most possible is your thermostat is stuck open. With a new one it shouldn't but who knows. If my memory is correct Wahler is OEM manufacturer of the BMW thermostat. The original factory aluminum thermostat I had replaced on my car had Wahler brand on it, along with BMW logo. So it should be good, but you never know. If you have warranty on the part ask for replacement. Maybe you got a dud or a returned already one if it is a shady online seller.

The other possibility is one of the coolant temperature sensors is not working well, but that is lower possibility since those sensors are vey simple elements. There is one sensor right close to the oil filter housing, towards the front of the engine.
The other coolant temperature is on the hose at the passenger side of the radiator, you will see a wire going to it from top.
By the way I don't recommend non-OEM replacement of these sensors, unless you want to try them for troubleshooting.

[estrellajon]

Thanks for the reply. I did check all harnesses and all good. The first thermostat I used was mahle and after 6k Miles , light came back on. I then purchased a Wahler brand from ECS Tuning along with a brand new OEM coolant. After 300 Miles , error lit up

Now that you mentioned the other coolant switch. I also replaced that the first time around but I used Febi brand. Using non OEM could be a possibility?

Also what switch on the bottom hose are you referring too. Fan switch?

[PhaseP]

Originally Posted by estrellajon

Thanks for the reply. I did check all harnesses and all good. The first thermostat I used was mahle and after 6k Miles , light came back on. I then purchased a Wahler brand from ECS Tuning along with a brand new OEM coolant. After 300 Miles , error lit up

Now that you mentioned the other coolant switch. I also replaced that the first time around but I used Febi brand. Using non OEM could be a possibility?

Also what switch on the bottom hose are you referring too. Fan switch?

Temperature sensors, not switches to be more accurate.

Going by my car model and year, one is number 10 "Temperature sensor water/oil" in this link, which measures coolant temperature at engine :

http://www.realoem.com/bmw/enUS/show...diagId=11_3736

The other one is number 3 "Temperature sensor, coolant" on this link, right at the hose connection that is coming off of the radiator:

http://www.realoem.com/bmw/enUS/show...diagId=17_0350

This later one is measuring the coolant temperature after it is cooled down by the radiator.

I had bought non-BMW of both of these sensors two years or so back when I was chasing a hard to troubleshoot error and used for a while. Then put the originals back into the car, and still have the originals on the car.

Out of curiosity I had once tested and measured them with boiling water and icy water against the originals from factory. The non-BMW ones I had bought were reporting very different values than the originals. That is why I don't recommend them. The BMW ones are ridiculously expensive though for what they are, as usual you pay for the logo.
Things may have changes since then. When I had bought those sensors there were not even Febi brands. Some off brands, one was "O.E.M" brand