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Old 05-25-2021, 09:31 AM   #1
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2002 L-Series 3.0L Sedan
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Default Temperature Gauge Issue.

2008 2.4 . 160K miles. Temperature gauge will start to climb normally. When it reaches around 140 F the gauge drops to zero. Will not show any temp thereafter. Even after letting the car sit and then restarting it. Disconnecting/reconnecting the battery results in the gauge initially working then dropping to zero. Changed the ECT, still does the same thing.

Any ideas ?
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Old 05-25-2021, 10:40 AM   #2
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Default Re: Temperature Gauge Issue.

I should also note that when the gauge stops reading. The fans turn on and the A/C wont run
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Old 05-25-2021, 11:10 AM   #3
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Default Re: Temperature Gauge Issue.

Plug in a reader to see coolant temps. Since the OBD II gets info from the bcm, the ecm sends coolant signals to the bcm. The bcm drives the temperature gauge and sends the same info to the reader. This may help diagnose which way to go.

In (presumed) electrical logic, the coolant sensor connects to the ecm directly and sends temperature signals to the bcm. The bcm drives all the gauges, interior and exterior lights, processes security, and is the gateway for OBD II signals to a reader or scantool.
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Old 05-25-2021, 01:09 PM   #4
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Default Re: Temperature Gauge Issue.

Should have noted that we did that. The reader shows coolant temp even when the gauge doesnt.
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Old 05-25-2021, 02:20 PM   #5
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Default Re: Temperature Gauge Issue.

Without service manual info, I'm guessing based on presumed info of electrical wiring. With a reader always displaying coolant temps while the gauge dies, the bcm, I believe, sends signals to the gauge to move the needle. Temperature signals should flow from ecm to bcm to instrument panel gauge but as serial data in a pipeline along with other signals. If I'm not mistaken, a chip in the instrument panel cluster (IPC) receives bcm signals then disseminates them to each gauge and indicator. If the chip fails, the ipc is replaced. Swapping ipc is one way to see if the ipc has the failure. All signals are sent as digital data in a serial pipeline where the ipc chip separates then sends data to each gauge and indicator. My guess is partial failure of this ipc chip. This presumes all indicators and gauges are operating normally with the exception of the intermittent temperature gauge. There's a possibility of the temperature gauge circuit being intermittent. Isolating ipc issues from bcm issues, I believe, is on an electronic level of serial communications thru chip circuitry.
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Old 05-25-2021, 04:42 PM   #6
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Default Re: Temperature Gauge Issue.

Thanks for your insight. I will report back
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Old 05-25-2021, 05:49 PM   #7
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Default Re: Temperature Gauge Issue.

The fail safe mode, to protect the engine, when the ECM deems the temperature measurement to be unreliable is to run the fans and stop the AC from operating. This is sometimes caused by faulty sensor, sensor wiring or thermostat. Someone in a thread here found that the rubber gasket was bad and was allowing coolant to bypass the thermostat even though the thermostat was functional. This caused the engine to never reach operating temperature.
There would normally be a code set for this condition, usually P0128, you didn't mention if you read any codes. You also didn't say what the temperature values that you read were. You should get something around 200 degrees f when the engine is warmed up and thermostat and sensor are working properly. The thermostat being mechanical is independent of everything else. If you measure a lower temp on a reader, suspect the thermostat. The temp gauge in the IPC likely doesn't even start to move until the temp is over 120 maybe 140 degrees.
https://www.700r4transmissionhq.com/p0128-saturn-aura/
The IPC is not capable of controlling the fans or AC. As fdryer says, it translates the digital temperature reading to the analog gauge display. If you were to swap out the IPC with another...the odometer will no longer count up and it cannot be programmed, even by a dealer. This I know from my own experience.
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Old 05-25-2021, 05:52 PM   #8
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Default Re: Temperature Gauge Issue.

My money is on the thermostat, opening normally during warm up and then sticking open until the engine cools down completely.
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Old 05-25-2021, 07:16 PM   #9
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Default Re: Temperature Gauge Issue.

I just went and did some testing on my Aura. The sensor reading showed 64 degrees before I started the engine. I was off a bit...the temperature gauge just started to lift when the sensor was reading around 95 degrees. Once the engine was running at operating temperature the sensor was reading around 190 degrees.
Here is a pic of the gauge at ~190 degrees...
Attached Images
File Type: jpg IMG_20210525_200354_592.jpg (211.4 KB, 3 views)
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Old 05-26-2021, 09:18 AM   #10
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Default Re: Temperature Gauge Issue.

Yes we had a P0128 code. The temperature didnt reach normal while we were monitoring it. Got to about 150 and stayed there. The thermostat was put in less than a month ago. We took it out and discovered it was stuck open. Replaced the thermostat and all is well.
Still dont understand why a stuck open thermostat caused the gauge to stop reading and the fans to turn on.
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Old 05-26-2021, 09:37 AM   #11
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Default Re: Temperature Gauge Issue.

When the coolant temperature doesn't reach operating temperature within a preset amount of time The ECM is programmed to consider the sensor reading as potentially bogus. It has no way to know whether there is a bad sensor or thermostat. When this condition occurs the program is designed to err on the side of caution and turn on the fans and disable the AC to reduce the load on the engine.
My Aura has the 3.5l engine. I'm not sure if the gauge calibration is different for your 2.4l or it could be that when the ECM detects temperature problems, the programming may stop sending data to the gauge.
For future reference, It might be worth your while to monitor the temp gauge while watching the sensor reading number on your scanner so you will know when the gauge needle begins to lift and what a normal warmup looks like.
From my test the gauge needle in my cluster started to move at about 95 degrees.
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Old 05-26-2021, 10:10 AM   #12
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Default Re: Temperature Gauge Issue.

OK, thanks for the info
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Old 05-26-2021, 11:49 AM   #13
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Default Re: Temperature Gauge Issue.

I doubt the thermostat is killing the temperature gauge when it drops out. They're likely two separate issues, P0128 and the recurring temperature gauge dropout. When the P0128 error code appeared, replacing the t-stat was the correct repair - no more error code as the new t-stat is operating as designed to show full engine warm-up. This should appear in your reader from cold startup with coolant temps showing close to outside temps them warming up until reaching around 195F when it starts regulating coolant flow for normal engine running. Your reader should display operating temperatures throughout a drive cycle. Since you did not mention hearing/seeing the cooling fan(s) running or lack of ac when turning it on, either this was missed and occurred or not.

If you can, note when the temperature gauge drops out with your reader displaying actual coolant temps. I'm presuming you're seeing coolant temps change from engine startup to fully warmed engine. Relying on reader temperature displaying coolant temps is helpful in diagnosing where the signals are lost and/or whether or not the ipc or bcm are having partial failure. The trick is having reliable data wherever it comes from and working from there.
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Old 05-26-2021, 12:21 PM   #14
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Default Re: Temperature Gauge Issue.

I'll just leave this here:
https://www.autocodes.com/qa/24450/2...-what-solution
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Old 05-26-2021, 12:24 PM   #15
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Default Re: Temperature Gauge Issue.

Here is same issue on a 2.4l Malibu...
https://www.chevymalibuforum.com/thr...ge-fail.12036/
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Old 05-26-2021, 12:32 PM   #16
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Default Re: Temperature Gauge Issue.

Since replacing the thermostat the fans and A/C work as they should
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Old 05-26-2021, 12:44 PM   #17
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Default Re: Temperature Gauge Issue.

Quote:
Originally Posted by lrbraner View Post
Since replacing the thermostat the fans and A/C work as they should
GM in all their wisdom chose to not light the check engine light right away on a P0128 (the code has to re-appear for a set number of driving cycles before turning on the Check Engine light), but they also chose to also disable the temp gauge reading when the code is pending. Makes troubleshooting, what would otherwise be simple, all the harder.
Why did they do it...Only GM knows for sure...lol.
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Old 05-26-2021, 03:24 PM   #18
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Default Re: Temperature Gauge Issue.

Apparently GM may have altered(?) programming relative to P0128. With similar symptoms from the Aura and Malibu links, killing the temperature gauge, turning on cooling fans before replacing the faulty t-stat fixed the symptoms. I'm not sure why replacing the coolant sensor was the first choice instead of t-stat without a reader to monitor coolant temps. However, a faulty coolant sensor may or may not give false temperature readings. Separating a faulty coolant sensor from faulty t-stat, other than using a reader, might require a plain thermometer into the coolant container or ir hand held probe to measure coolant temps.

When my L300 triggered P0128, I read my service manual with zero descriptions about turning on cooling fans and disabling ac. In my situation, this occurred in winter temps so it never occured to me to check for fans or ac when outside temps were near freezing and heater use was needed. I simply reset the error code, not worried about it until warm spring weather when it reappeared. I wasn't concerned about the temperature gauge freezing in place and it appeared to move normally albeit stopping at a lower position coinciding with slightly lower reader display of coolant temps. I recall using a reader to monitor temps from startup to see what actual temps were, even presuming a faulty sensor or t-stat. I think I monitored cold engine temps compared to outside then continued monitoring as the engine reached operating temps to rule out a sensor fault. I saw a little less than ideal operating temps on the gauge and reader display, and concluded a t-stat fault. It's much easier to replace the coolant sensor but very tedious to replace the t-stat. The t-stat is buried in the V section of 3.0L engines with several hours needed to remove upper engine parts before the t-stat becomes visible. If it were as simple as replacing the coolant sensor, I would have but everything led me to believe it was the t-stat. I still had sufficient heat in winter so I wasn't concerned until warmer temps allowed in garage repairs. A similar P0128 occurred to a Nissan Sentra in summer with zero fans or ac disabled. Monitoring temps before and after replacing the t-stat corrected it.

Last edited by fdryer; 05-26-2021 at 03:31 PM.
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Old 05-26-2021, 08:22 PM   #19
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Default Re: Temperature Gauge Issue.

More info, in case anyone else comes looking for solutions behind us...hopefully it will shed light on the temp gauge issue caused by P0128 and help someone else quickly diagnose the cause as the thermostat rather than going down the rabbit hole of trying to diagnose a faulty temperature gauge or sensor.

Pontiac G6 P0128 program works same way, cuts off temperature gauge when P0128 is set.
There are some troubleshooting steps and criteria for setting the code in this thread.
https://www.g6ownersclub.com/threads...0-no-ac.50835/

More details and steps in this link:
"In many GM cars and trucks (Chevrolet Trailblazer, Silverado, HHR and other models) the code P0128 is often caused by a sticking thermostat. In some GM cars when the code P0128 is set, the temperature gauge stops working and the radiator fans run constantly. Replacing the thermostat often solves the problem unless there are some other issues.
The GM TSB dated 2007 recommends reprogramming the PCM as a solution for the code P0128 in 2006 Chevrolet Malibu, Impala, Pontiac G6, and few other models."
https://www.samarins.com/diagnose/p0128.html
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Old 08-21-2021, 10:12 PM   #20
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Default Re: Temperature Gauge Issue.

Great thread with lots of helpful links.

My 09 Aura 2.4 with ~80k has intermittently gone through bouts of P0128.

The latest CEL was cleared with A/C functionality and radiator fans returning to normal operation when I pulled and immediately re-inserted 3 or 4 fuses from the fuse box that I figured could be related to ECT/Thermostat/Coolant, etc.

After driving for about 90 miles or so, the temperature gauge dropped to the lowest value, and the radiator fans came back on full blast. The CEL hasn't come back on, but I suspect it will the next time I start my car.

My question is since the problem was resolved after fuses were untoggled/toggled, would that point to an electrical wiring issue, or does it change nothing w/r/t the usual diagnosis (i.e. ECT and/or Thermostat)?
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