2EF4 - Thermostat code

Gizm0 · 01-19-2026, 08:54 AM

A couple months ago, I replaced the thermostat on my N54 because of a code 2EF4, and the water temp would hover around 160/170 degrees F (not the oil temp, but the water temp). Oil temps would also be a little cooler than normal. After replacing the thermostat, problem was solved...for a few weeks, then it came back.

I checked the radiator upper and lower hose temperatures with a thermal camera during a cold start to confirm this, and I could clearly see, while the car was warming up, the lower hose temp was steady at around 30/40F, while the upper hose temp rose up to about 165/170, then the thermostat opened, allowing flow through the radiator and the lower hose temp quickly rose over 120F.

I think my mistake was to buy a cheap Amazon $30 thermostat (4.7 star reviews so I thought I could trust it - brand is MITZONE), and the wax pellets in the thermostat are just low quality and degraded quickly, so I want to replace it with one from FCP euro, but before I do that, has anyone experienced the same symptoms? I just find it weird that it's mimicking the exact same symptoms my car was doing before I replaced the previous thermostat... I get it, buying cheap Chinese knock off parts for a BMW is never a good idea, but it's strange that it's doing the exact same thing it did before... especially since it worked fine for a few weeks then suddenly came back. I'm thinking it might be something electrical where the DME is sending voltage to the thermostat to open it at a lower temperature than what it's supposed to. I know with this car, when engaged in spirited driving, the DME will send voltage to the thermostat heating element to open it to help lower water temps for increased performance/engine protection, and during normal/easy driving, it doesn't send voltage so the thermostat only opens when the wax element reaches about 205F, for increased fuel economy and efficiency... If it was a bad coolant temp sensor then the computer would show the water temp being higher than what it actually is, however, the thermal camera readings were within 5 degrees of what the computer showed for water temps...

mainbearing · 01-19-2026, 11:16 AM

BMW cooling system is kinda mediocre to begin with. Such an important part you should use only genuine BMW or OEM. The coolant tank vent hose I did use Amazon, as were the exterior trunk switch and cup holders. Those worked fine.

I would suspect the Amazon thermostat before the DME. Also, could it be the water pump going out?? I do not know the details of the data bus system, but maybe they are on the same data bus?

I used a Behr thermostat and Pierburg pump for N51, even though the quality of Behr cooling system products seem to be going downhill (a reservor tank developed 2 pinhole leaks after a couple years). OEM thermostat from BMW might be made by Wahler. The reason I did not get that was because reviews of leaks. Behr worked fine, so next time I think I will probably still use it with a Pierburg pump.

Gizm0 · 01-22-2026, 07:53 AM

Quote:

Originally Posted by mainbearing

BMW cooling system is kinda mediocre to begin with. Such an important part you should use only genuine BMW or OEM. The coolant tank vent hose I did use Amazon, as were the exterior trunk switch and cup holders. Those worked fine.

I would suspect the Amazon thermostat before the DME. Also, could it be the water pump going out?? I do not know the details of the data bus system, but maybe they are on the same data bus?

I used a Behr thermostat and Pierburg pump for N51, even though the quality of Behr cooling system products seem to be going downhill (a reservor tank developed 2 pinhole leaks after a couple years). OEM thermostat from BMW might be made by Wahler. The reason I did not get that was because reviews of leaks. Behr worked fine, so next time I think I will probably still use it with a Pierburg pump.

Thanks! I bought a replacement from FCP Euro. Out of curiosity, i am going to disconnect the thermostat harness and go for a drive to confirm if the DME for some reason sending 12V when water temps reach 170. If water temps are normal with thermostat unplugged, that would mean it is indeed sending 12V when it's not supposed to. Most likely the thermostat is the culprit though but running this test is easy.

mainbearing · 01-24-2026, 04:59 PM

You can check out this wiring diagram from the electrical expert gbalthrop here. The control circuit power for both the pump and thermostat look joined (orange wire. The N54 400W (?) pump motor looks to get its power from the red wire). Looks like with key on/engine off, there should be battery voltage at both harness connectors.

However, DME turns on/off the thermostat on the Ground side.

https://www.e90post.com/forums/showthread.php?t=1896342

01-19-2026, 08:54 AM	#1
Gizm0 Colonel 550 Rep 2,256 Posts Drives: 08 e92 335i 6mt Join Date: Jan 2013 Location: Virginia iTrader: (1)	2EF4 - Thermostat code - Engine running cold A couple months ago, I replaced the thermostat on my N54 because of a code 2EF4, and the water temp would hover around 160/170 degrees F (not the oil temp, but the water temp). Oil temps would also be a little cooler than normal. After replacing the thermostat, problem was solved...for a few weeks, then it came back. I checked the radiator upper and lower hose temperatures with a thermal camera during a cold start to confirm this, and I could clearly see, while the car was warming up, the lower hose temp was steady at around 30/40F, while the upper hose temp rose up to about 165/170, then the thermostat opened, allowing flow through the radiator and the lower hose temp quickly rose over 120F. I think my mistake was to buy a cheap Amazon $30 thermostat (4.7 star reviews so I thought I could trust it - brand is MITZONE), and the wax pellets in the thermostat are just low quality and degraded quickly, so I want to replace it with one from FCP euro, but before I do that, has anyone experienced the same symptoms? I just find it weird that it's mimicking the exact same symptoms my car was doing before I replaced the previous thermostat... I get it, buying cheap Chinese knock off parts for a BMW is never a good idea, but it's strange that it's doing the exact same thing it did before... especially since it worked fine for a few weeks then suddenly came back. I'm thinking it might be something electrical where the DME is sending voltage to the thermostat to open it at a lower temperature than what it's supposed to. I know with this car, when engaged in spirited driving, the DME will send voltage to the thermostat heating element to open it to help lower water temps for increased performance/engine protection, and during normal/easy driving, it doesn't send voltage so the thermostat only opens when the wax element reaches about 205F, for increased fuel economy and efficiency... If it was a bad coolant temp sensor then the computer would show the water temp being higher than what it actually is, however, the thermal camera readings were within 5 degrees of what the computer showed for water temps... __________________ JB4 G5 w/ Backend flash - Walbro 450lpfp - VRSF DPs - VRSF 5" stepped IC - DCI - ER CP w/ Tial BOV - 335is clutch Last edited by Gizm0; 01-19-2026 at 01:52 PM..
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01-19-2026, 11:16 AM	#2
mainbearing Colonel 1749 Rep 2,165 Posts Drives: BMW 328i Join Date: Apr 2019 Location: LA iTrader: (0)	BMW cooling system is kinda mediocre to begin with. Such an important part you should use only genuine BMW or OEM. The coolant tank vent hose I did use Amazon, as were the exterior trunk switch and cup holders. Those worked fine. I would suspect the Amazon thermostat before the DME. Also, could it be the water pump going out?? I do not know the details of the data bus system, but maybe they are on the same data bus? I used a Behr thermostat and Pierburg pump for N51, even though the quality of Behr cooling system products seem to be going downhill (a reservor tank developed 2 pinhole leaks after a couple years). OEM thermostat from BMW might be made by Wahler. The reason I did not get that was because reviews of leaks. Behr worked fine, so next time I think I will probably still use it with a Pierburg pump.
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01-24-2026, 04:59 PM	#4
mainbearing Colonel 1749 Rep 2,165 Posts Drives: BMW 328i Join Date: Apr 2019 Location: LA iTrader: (0)	You can check out this wiring diagram from the electrical expert gbalthrop here. The control circuit power for both the pump and thermostat look joined (orange wire. The N54 400W (?) pump motor looks to get its power from the red wire). Looks like with key on/engine off, there should be battery voltage at both harness connectors. However, DME turns on/off the thermostat on the Ground side. https://www.e90post.com/forums/showthread.php?t=1896342
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