Tempurature Gauge Sudden Failure and No A/C

Benson86

Original poster
Member
Mar 15, 2020
12
Clearwater FL
Hi all, new issue on the 08 9-7x 4.2 came up yesterday. I'm already trying to diagnose some electrical issues in this thread here, but wanted to start a new thread for this as it could be a totally new issue. Have been driving the car for about week now and its been driving fine, temp has been showing at 12-1 oclock on the temp gauge every day and car has been driving normal. Yesterday the temp gauge suddenly dropped to 0, I got the light saying check the gauges and then after an hour of driving I got a CEL. Pulled the code and its P0128 Coolant Temp Low, but i find that hard to believe when it was perfect that something failed that fast. The only thing I havent done yet is use my scanner to see that actual temp being reported but wil do that tomorrow. Could this be realted to my electrical issues somehow? Thanks!
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
As mentioned in the other thread, likely the coolant temp sensor is defective. The PCM is going according to the sensor. Don't run it like that for too long or you'll destroy the cat because it is likely running in open loop and burning extra fuel. It dropped all of a sudden so that would point to the sensor suddenly dying or the wiring.
 
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Benson86

Original poster
Member
Mar 15, 2020
12
Clearwater FL
As mentioned in the other thread, likely the coolant temp sensor is defective. The PCM is going according to the sensor. Don't run it like that for too long or you'll destroy the cat because it is likely running in open loop and burning extra fuel. It dropped all of a sudden so that would point to the sensor suddenly dying or the wiring.

The threads I did find on the issue people replaced the sensor and had it not resolve the issue, one mentioned the stepper motor for the gauge. Ive been researching all day and I've found plenty of people that have had the issue, and it always causes the A/C to go out for some reason, but no one has ever posted what actually fixed it :/

Will be doing more diagnosing with the scanner tomorrow. Im not driving it now for just that reason.
 

m.mcmillen

Member
Apr 29, 2016
554
Wisconsin
If your coolant temperature is showing zero, there is probably an open in the circuit somewhere (probably in the sensor). If you unplug the sensor and check the resistance of the sensor across the two terminals, there should be something there. If the circuit reads open, the sensor is your problem.
 
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Benson86

Original poster
Member
Mar 15, 2020
12
Clearwater FL
If your coolant temperature is showing zero, there is probably an open in the circuit somewhere (probably in the sensor). If you unplug the sensor and check the resistance of the sensor across the two terminals, there should be something there. If the circuit reads open, the sensor is your problem.

Thats easy enough to test, is the sensor relatively easy to get at?
 

Benson86

Original poster
Member
Mar 15, 2020
12
Clearwater FL
Update, when I got home I unplugged the negative terminal on the battery for 30 sec and hooked it bakc up. Temp gauge fired right back up and all the lights went away. Took a quick 10 min drive and temp got up to about 11 oclock on the gauge. My scanner wont show me the actual temps. Im wondering about the tstat because should 10 min pretty much get the coolant all the way to the middle of the gauge? Doesn't seem like it would be the sensor if a battery reset gets it working again, just seems electrical to me.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
08-09 it's on the right side of the head at the rear.

A failed stepper would not cause a code nor a check gauges message and they normally would not just drop to 0. They're usually all over the place when they go.
 

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budwich

Member
Jun 16, 2013
2,027
kanata
what does this mean? "My scanner wont show me the actual temps." Does that mean it can only read codes or that it reads "0". IF the data reads "0", there is a reason for this (likely no input measurement is possible). As for the Tstat, it could be faulty in terms of stuck open BUT that will not stop a temperature reading.
 
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Benson86

Original poster
Member
Mar 15, 2020
12
Clearwater FL
what does this mean? "My scanner wont show me the actual temps." Does that mean it can only read codes or that it reads "0". IF the data reads "0", there is a reason for this (likely no input measurement is possible). As for the Tstat, it could be faulty in terms of stuck open BUT that will not stop a temperature reading.

As far as I can figure it will only read codes. Its a small little orange auto parts store one that ive had for 5-6 years. Ill download the manual today and see if im missing something. But wouldn't the fact that re-plugging the battery got the gauge to work normally again mean the sensor at least is ok?
 

budwich

Member
Jun 16, 2013
2,027
kanata
the sensor may or may not be an issue and its place and likely "stuck in place mode" on the engine indicates to me not to go there (ie. removal). The wiring / connector is a potential source for a problem. IF it is questionable then its readings are also suspect for both the display and the engine running controller. The code is a measurement issue.... only you can determine where the problem lies. At worst, follow the circuit diagram to where the "pair" ends, pull the connector at that end and do your resistance measurement there. The posted tables should tell you how close or how far the measuring circuit is... go from there.
 

TJBaker57

Member
Aug 16, 2015
2,897
Colorado
Might be useful to be aware of all the conditions required for that P0128 to set. Also note that it is a type B DTC. As far as I know type B DTC do not turn on the check engine light until it happens twice consecutively.

Screenshot_20200503-103348.png
 

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