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      03-24-2021, 08:04 AM   #1
Letsgo
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Diagnosing: Thermostat vs. Coolant Temp Sensor

I bought a 2016 X5 35i with ~75k miles roughly a month ago. I had pre purchase inspection done at the dealer, the bmw master tech showed me the Control unit tree which was all green at the time, no issues.

Driving yesterday I noticed my engine temp remain at the lowest setting without increasing as I drove, no heat coming into the car and a loud noise from outside, which ended up being my fan running at full speed during my entire drive of about 15 minutes. FYI driving home the fan didn't turn on, however I did get a CEL, and the engine temp was very slow to raise and it never got to normal operating temp.

Got home and ran my scans, I will start with the coolant temp sensor. Any feedback or thoughts if I should be aware of anything else. Could this potentially be a thermostat issue as well?

Fault Code: Thermostat jammed open.


OBD2 Codes:
U1408: (can't find definitive for what this is)
P0128: Coolant temp

For some odd reason I am also getting something about the battery being old, which is odd. If I don't drive the car for a week I usually have it hooked up to an 'intelligent' trickle charger.









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      03-24-2021, 10:41 AM   #2
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Keep in mind that temperature on the dash is not the coolant, it is the oil. I would try to clear all errors in ISTA, drive around the block and see what comes back.
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      03-24-2021, 12:39 PM   #3
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Thanks for the heads up, makes sense.

Is there a way to show operating temp?

I ordered the sensor, will install, clear codes and post update here.
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      03-24-2021, 12:54 PM   #4
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Quote:
Originally Posted by Letsgo View Post
Thanks for the heads up, makes sense.

Is there a way to show operating temp?

I ordered the sensor, will install, clear codes and post update here.
Maybe try thru the hidden menu?

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      04-14-2021, 07:22 PM   #5
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Quick update, I replaced the coolant temp sensor with an OEM BMW one ($50). Replaced it, cleared the codes and drove trouble free for the last 3 weeks (1,000km).

CEL came on again, same two codes:

OBD2 Codes:
U1408: (can't find definitive for what this is)
P0128: Coolant temp

Will delete again and report back.

If it comes back I guess the thermostat will be next?

FYI the engine oil temp gauge is functioning normal and I don't have the fan coming on excessively like it did originally.
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      04-14-2021, 08:01 PM   #6
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Check this thread out:
https://www.bimmerfest.com/threads/s...-help.1317145/
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      04-15-2021, 05:22 AM   #7
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Quote:
Originally Posted by RED_Y_ View Post
Thanks for the link, this is helpful.

To keep documenting in this thread, the linked thread has a number of people with the same problem who replaced thermostats and the problem didn't go away. A poster did state this however which is promising:

"For me they first replaced the MAP thermostat PN 11-53-8-635-689. That did nothing and the car still wasn't coming up to operating temp in time. The oil temp would hang at 70C on the bottom end of the gauge - even after a 30-40 min drive. I also kept getting intermittent check engine lights for P0128 and U1408 codes.

I took it back to the dealer and they replaced the auto transmission oil cooler (heat exchanger) PN 17-21-7-600-553. There is second thermostat on this part that went bad. That solved the problem. My car now comes up to operating temp after a 10 min or so drive on the highway - even in very cold weather.

I can't find it now, but if you search the internet you will see evidence that this is a somewhat common problem."



Also I wonder if this is true (from another poster):

"There***8217;s an extended warranty for thermostats stuck open 10 years 120,000 miles"
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      05-06-2021, 09:46 AM   #8
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Not bad, made it another 3 weeks without a CEL, another ~1,000 km and they are back.
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      05-06-2021, 10:02 AM   #9
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Did you have the transmission cooler replaced under warranty?
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      05-06-2021, 01:54 PM   #10
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Should have bought a diesel
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      05-06-2021, 11:50 PM   #11
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Quote:
Originally Posted by blue dragon View Post
Should have bought a diesel
Thermostats never fail on gasoline car?
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      05-07-2021, 12:20 PM   #12
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Quote:
Originally Posted by RED_Y_ View Post
Thermostats never fail on gasoline car?
Sorry, inside joke. I know the OP personally
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      08-03-2021, 09:14 AM   #13
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Any update?

Quote:
Originally Posted by Letsgo View Post
Not bad, made it another 3 weeks without a CEL, another ~1,000 km and they are back.
I just got the exact two codes this morning, poor acceleration, fans on full speed. Did you get this fully resolved, if so what was the resolution?
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      10-07-2021, 09:59 PM   #14
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      02-11-2023, 04:08 PM   #15
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I'm having the same issue. Error code P0128. Did any of you guys get this resolved?
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      03-05-2023, 06:19 AM   #16
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I changed the coolant temp sensor and I haven't had any issues since.
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      03-07-2023, 06:48 PM   #17
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Quote:
Originally Posted by Letsgo View Post
Quick update, I replaced the coolant temp sensor with an OEM BMW one ($50). Replaced it, cleared the codes and drove trouble free for the last 3 weeks (1,000km).

CEL came on again, same two codes:

OBD2 Codes:
U1408: (can't find definitive for what this is)
P0128: Coolant temp

Will delete again and report back.

If it comes back I guess the thermostat will be next?

FYI the engine oil temp gauge is functioning normal and I don't have the fan coming on excessively like it did originally.
Looks like the CEL came back on after you replaced the coolant temp sensor. So did you clear the codes again and haven't had a CEL since?
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      03-07-2023, 10:23 PM   #18
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Buy a Water Pump Replacement Kit from FCPEuro. Install it. Problem fixed.
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      03-09-2023, 10:56 AM   #19
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Quote:
Originally Posted by Tomcat_F15_STL View Post
Buy a Water Pump Replacement Kit from FCPEuro. Install it. Problem fixed.
That's exactly what I did. Crazy thing is I did an oil change right before the kit showed up and the CEL vanished. Radiator fan quit working overtime and oil temp start to rise to normal. Drove the car 100 or so miles and then just like I suspected it triggered another CEL. I'll get the t-stat and waterpump replaced next week.
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      03-09-2023, 07:08 PM   #20
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Quote:
Originally Posted by M302_imola View Post
That's exactly what I did. Crazy thing is I did an oil change right before the kit showed up and the CEL vanished. Radiator fan quit working overtime and oil temp start to rise to normal. Drove the car 100 or so miles and then just like I suspected it triggered another CEL. I'll get the t-stat and waterpump replaced next week.
Best of luck. I’m confident those parts will fix you up.


BMW knows how to make some over-complicated shit! This includes the entire cooling system.
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      03-09-2023, 08:23 PM   #21
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Quote:
Originally Posted by Tomcat_F15_STL View Post
Best of luck. I’m confident those parts will fix you up.


BMW knows how to make some over-complicated shit! This includes the entire cooling system.
Looks under the hood , closes hood slowly whilst in utter panic.
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      03-09-2023, 11:41 PM   #22
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Quote:
Originally Posted by Chilled View Post
Looks under the hood , closes hood slowly whilst in utter panic.
That's funny.
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