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Old 10-31-2019, 02:15 PM
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Coolant sensor replacement

My SES light keeps lighting up throwing a P0128 code on my 2006 E-53 N62 engine. The code is directing me to a stuck thermostat. I can re-set the code and it is fine for a couple of days, then SES comes back on with the same code. There are no other underlying codes. This has been happening for several weeks now. The engine runs just fine indicating normal temp, no apparent leaks or exhaust issues, clear/clean oil, and strangely a slight improvement in fuel economy. I have not received a low coolant level warning on my display. I suspect a faulty temperature sensor and would like to replace it since it is relatively inexpensive. Even though I have normal temperature indications, I read where this sensor also controls the operation of the thermostat. I would appreciate some pointers if anyone has replaced this coolant sensor before. I would specifically like to know (with a cold engine), if can I simply unclip the wire connector and remove and replace the sensor without draining the coolant? (I have done this while replacing a lower heater element on a hot water heater and I only lost about a cup of water in the process. No need to drain the tank). I was wondering if this makes sense or am I setting myself up for a catastrophe? I would appreciate some advice. Thanks.
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Last edited by JNMNL52; 11-01-2019 at 03:41 PM.
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Old 10-31-2019, 03:02 PM
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I think you are barking up the wrong tree.
Your car may not in fact be losing coolant etc etc but, it may be taking too long to heat to the specified temp it's looking for.
This would set off the light.
Your temp sensor is not misreporting as you stated.
Change your thermostat.

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Old 10-31-2019, 05:28 PM
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Quote:
Originally Posted by zukgod1 View Post
I think you are barking up the wrong tree.
Your car may not in fact be losing coolant etc etc but, it may be taking too long to heat to the specified temp it's looking for.
This would set off the light.
Your temp sensor is not misreporting as you stated.
Change your thermostat.

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Thanks for coming up on the net and responding. My engine warms up to operating temperature normally within minutes and remains there so my sense is the thermostat is working ok. I thought it was prudent to replace the coolant sensor first to see if that resolved the issue before proceeding on to the thermostat-which is next on my list. Seems easy enough to do, especially if I do not need to totally drain the coolant.
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Old 10-31-2019, 06:26 PM
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You need to see the realtime ect (engine core temp aka coolant outlet temp). You can use a good scanner or use the hidden OBC menu.

The temp indicator on the dash is not remotely a "gauge" and reads noon from about 60 to 110 C so it is positively useless sending false sense of security most of the time.

Watch realtime temp and it should climb quickly to about 90-92C than stay right there. If it doesn't: stuck thermostat is the problem.
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Old 10-31-2019, 07:14 PM
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Originally Posted by andrewwynn View Post
You need to see the realtime ect (engine core temp aka coolant outlet temp). You can use a good scanner or use the hidden OBC menu.

The temp indicator on the dash is not remotely a "gauge" and reads noon from about 60 to 110 C so it is positively useless sending false sense of security most of the time.

Watch realtime temp and it should climb quickly to about 90-92C than stay right there. If it doesn't: stuck thermostat is the problem.
Hi Andrew: Thank you; very helpful. I would like to pursue. How do I access the hidden OBC menu to see the actual temperature?
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Old 10-31-2019, 07:57 PM
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There are numerous threads and Google pages how to get into the hidden menu. It's different if you habe the high or low cluster.

Search e53 hidden menu. There is a little bit of trick with holding the odometer button and turning on the key. Easy once you learn the tridk.
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Old 11-01-2019, 12:07 PM
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Originally Posted by andrewwynn View Post
You need to see the realtime ect (engine core temp aka coolant outlet temp). You can use a good scanner or use the hidden OBC menu.

The temp indicator on the dash is not remotely a "gauge" and reads noon from about 60 to 110 C so it is positively useless sending false sense of security most of the time.

Watch real time temp and it should climb quickly to about 90-92C than stay right there. If it doesn't: stuck thermostat is the problem.
Hi Andrew: I got into the hidden OBC menu this morning and checked the coolant temps. The outside air temp this morning was 44 degrees F. Here is what I indicated with engine just at idle (not racing):

Cold engine prior to start: 7C. The cluster temp gauge was pegged to the left in the blue paint at 10 o'clock. After start: 5 min-41C, 10 min-55C, 15 min-66C, 20 min-74C (temp gauge needle at 12 o'clock), 25 min-81C, 30 min-86C. After idling for 30 min I took the car out on the highway and steady state KTEMP was 90C for an hour commute to work.

I assume the 90C operating temp will occur a lot quicker if I started moving in the car after start - which is what I normally do and do not notice the temp gauge in the blue much after start. It appears that the temp sensor on the lower hose is providing accurate information to the thermostat, so I suspect the thermostat is stuck open given the amount of time it took to reach operating temp at idle but I do not have a feel for that. It did not climb quickly at idle as you stated. What are your thoughts?
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Old 11-01-2019, 02:58 PM
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When my thermostat was jammed open, it wasn't warm-up time that alerted me to the fact. Rather it was that the KTMP would fall when going down hills etc. Nomal running temp in my M57 is 88Deg.C. and I'd see the KTMP fall to 65Deg.C. when going down hills at highway speeds, even after the engine was (had been) to full temp. Too much cooling when the engine thermal load dropped off and the thermostat wasn't closing to maintain the KTMP.
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Old 11-01-2019, 03:35 PM
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Originally Posted by wpoll View Post
When my thermostat was jammed open, it wasn't warm-up time that alerted me to the fact. Rather it was that the KTMP would fall when going down hills etc. Nomal running temp in my M57 is 88Deg.C. and I'd see the KTMP fall to 65Deg.C. when going down hills at highway speeds, even after the engine was (had been) to full temp. Too much cooling when the engine thermal load dropped off and the thermostat wasn't closing to maintain the KTMP.
OK, thanks. It is pretty flat around here where I am and my KTEMP is staying at 90C +/-1. Probably unrelated but it is interesting to note that I noticed a slight improvement in my gas mileage since this issue started. I have been consistently reading 19.7 mpg in combined city and highway driving. Now indicating 19.9 mpg with no action on my part. Not bad for an N62 with 193,500 miles on it.
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Old 11-01-2019, 03:39 PM
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Quote:
Originally Posted by wpoll View Post
When my thermostat was jammed open, it wasn't warm-up time that alerted me to the fact. Rather it was that the KTMP would fall when going down hills etc. Nomal running temp in my M57 is 88Deg.C. and I'd see the KTMP fall to 65Deg.C. when going down hills at highway speeds, even after the engine was (had been) to full temp. Too much cooling when the engine thermal load dropped off and the thermostat wasn't closing to maintain the KTMP.
BTW, did you replace your thermostat yourself or did you have a shop replace it? It looks like about 3 hours of shop labor to me.
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