Coolant Thermostat Stuck Open

Pafolo · 11-16-2021, 05:43 PM

I got the code for a stuck thermostat according to ISTA+ and replaced the thermostat. The issue is I get the same exact code as before but with a replaced t stat. The old one wasn't stuck when I removed it and I doubt that the new one is bad.

Since I got the code before I replaced anything and I'm also getting the same code after replacing the T-Stat, I'm lead to believe that its not the T-Stat at all but something else that's triggering the code. I thought it could have been a bad coolant temp sensor but when I check the sensor it shows its working fine. I did a quick check of the wiring and nothing looks damaged.

Any Ideas what could be wrong or what to look into?

Has anyone else had this issue?

richardodn · 11-17-2021, 05:28 PM

Which engine? What's the code?

Pafolo · 11-17-2021, 06:13 PM

I believe its a p0128 or bmw code 00279B. The description within ISTA was

DME: Thermostat, map cooling: mechanism

Engine is a M54

richardodn · 11-18-2021, 09:28 AM

First of all, I assume you cleared the error code after the new thermostat was installed and it came back.

The mechanism that opens the thermostat is a wax motor. As the wax warms up it expands and opens the valve.

The thermostat opens in two different modes. There's the traditional method where the wax is warmed by the coolant. The other is where the wax is warmed by a heater circuit in anticipation of the engine reaching operating temperature. This is basically done by a lookup table aka map. The heater is controlled by the DME. Specifically, the heater gets power from fuse 2 (red/white wire) and the DME grounds the other wire (gray/brown).

The first thing I would do is to get live data from the DME coolant temp sensor so you can look at what is actually happening. Next I would disconnect the electrical connection from the thermostat and see if that behavior changes. It should still eventually open in the traditional mode. If it does change behavior, that indicates the either a wiring or DME is at fault. If it is the DME, it may be due to an input or an internal fault.

Pafolo · 11-19-2021, 10:37 PM

Ive ordered a replacement thermostat to warranty the new one just in case I received a dud. Ive already begun the electrical check on the wiring to see if its sending power to the heating element. I haven't tested all avenues when it comes to the wiring yet but I'm constantly thinking of each and every possibility that could be causing this issue.

11-16-2021, 05:43 PM	#1
Pafolo New Member 0 Rep 5 Posts Drives: E60 550i Join Date: Jan 2019 Location: Chicago iTrader: (0)	Coolant Thermostat Stuck Open I got the code for a stuck thermostat according to ISTA+ and replaced the thermostat. The issue is I get the same exact code as before but with a replaced t stat. The old one wasn't stuck when I removed it and I doubt that the new one is bad. Since I got the code before I replaced anything and I'm also getting the same code after replacing the T-Stat, I'm lead to believe that its not the T-Stat at all but something else that's triggering the code. I thought it could have been a bad coolant temp sensor but when I check the sensor it shows its working fine. I did a quick check of the wiring and nothing looks damaged. Any Ideas what could be wrong or what to look into? Has anyone else had this issue?
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11-17-2021, 05:28 PM	#2
richardodn First Lieutenant 160 Rep 347 Posts Drives: '12 X3 35iX/'20 440ix cabrio Join Date: Dec 2018 Location: Denver, CO iTrader: (0)	Which engine? What's the code? __________________ 1986 Delphin 528e - Roof rack equipped lumber hauler. 2008 Platinum-Beige X3 3.0si - Current project 2012 Alpine White X3 xDrive35i - My new snowmobile. 2020 Estoril Blue 440i xDrive cabrio w/MPPSK - This car is a blast to drive.
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11-17-2021, 06:13 PM	#3
Pafolo New Member 0 Rep 5 Posts Drives: E60 550i Join Date: Jan 2019 Location: Chicago iTrader: (0)	I believe its a p0128 or bmw code 00279B. The description within ISTA was DME: Thermostat, map cooling: mechanism Engine is a M54
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11-18-2021, 09:28 AM	#4
richardodn First Lieutenant 160 Rep 347 Posts Drives: '12 X3 35iX/'20 440ix cabrio Join Date: Dec 2018 Location: Denver, CO iTrader: (0)	First of all, I assume you cleared the error code after the new thermostat was installed and it came back. The mechanism that opens the thermostat is a wax motor. As the wax warms up it expands and opens the valve. The thermostat opens in two different modes. There's the traditional method where the wax is warmed by the coolant. The other is where the wax is warmed by a heater circuit in anticipation of the engine reaching operating temperature. This is basically done by a lookup table aka map. The heater is controlled by the DME. Specifically, the heater gets power from fuse 2 (red/white wire) and the DME grounds the other wire (gray/brown). The first thing I would do is to get live data from the DME coolant temp sensor so you can look at what is actually happening. Next I would disconnect the electrical connection from the thermostat and see if that behavior changes. It should still eventually open in the traditional mode. If it does change behavior, that indicates the either a wiring or DME is at fault. If it is the DME, it may be due to an input or an internal fault. __________________ 1986 Delphin 528e - Roof rack equipped lumber hauler. 2008 Platinum-Beige X3 3.0si - Current project 2012 Alpine White X3 xDrive35i - My new snowmobile. 2020 Estoril Blue 440i xDrive cabrio w/MPPSK - This car is a blast to drive. Last edited by richardodn; 11-18-2021 at 09:44 AM..
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11-19-2021, 10:37 PM	#5
Pafolo New Member 0 Rep 5 Posts Drives: E60 550i Join Date: Jan 2019 Location: Chicago iTrader: (0)	Ive ordered a replacement thermostat to warranty the new one just in case I received a dud. Ive already begun the electrical check on the wiring to see if its sending power to the heating element. I haven't tested all avenues when it comes to the wiring yet but I'm constantly thinking of each and every possibility that could be causing this issue. __________________ 2000 STAHLGRAU METALLIC - E46 323i sold with 252k miles 2006 HIGHLANDGRUEN METALLIC - E83 X3 3.0i 2007 SPACEGRAU METALLIC - E92 328xi Totaled 2008 PLATINGRAU METALLIC - E60 550i M pack 2011 BLACK SAPPHIRE METALLIC - F25 X3 35i M Pack
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