Temperature gauge goes inop p0128

vipergg

Well-Known Member
2008 Ascender , . No CEL currently . Temp gauge will go to zero at times and getting a P0128 at the same time . I find it interesting that the stat could cause this to happen . If that is the case it must be some stupid ECU programming that says when you see this code shut off the temp gauge . I have read in more than 1 place of people having the same issue and it even goes so far as to say if and when they cleared the code the gauge and AC started working again .
So the fix for this is the stat which is the normal for a p0128 code ? Anyone else had this problem and fix it with a new thermostat ? It could be the stat in my case , after driving it for 15 -20 minutes on my code reader which has live data the ECT was only reading about 175 degrees which is low I think for this vehicle . At this point I have cleared the codes to see if it comes back as it is supposed to be in the 20's again tomorrow and want to see if it comes back . So should I lean towards the stat and not say the ECT sensor ? I was getting a reading from ECT when the gauge said zero so that is a plus to rerplace the stat . Any insight appreciated.
 

Blckshdw

Moderator
If the temp sensor was saying 175 degrees, you're definitely low. Normal opening temperature is 195 degrees, so your t-stat is due for replacement. The t-stat is mechanical though, and won't cause the temp reading to drop off the map and go to zero, that would be the sensor, or a failing stepper motor. So to clarify, when the dash gauge dropped to zero, you still had a temp reading through the live data? If that's the case, then I'd say stepper motor for sure.
 
OP
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vipergg

Well-Known Member
Ran across this so I think it is the stat first , https://ricksfreeautorepairadvice.com/p0128-and-temperature-gauge-dead-gm-vehicles/ ,
If you get a P0128 code AND the temperature gauge is dead, head right for the thermostat. When a thermostat goes bad on a GM vehicle, the PCM sets a P0128 trouble code and commands the cooling fans to run and disables the temperature gauge. Don’t waste time trying to fix the gauge. First, replace the thermostat. If the gauge still doesn’t work, then you can start diagnosing that problem after you’ve cleared the P0128 code.
 

Blckshdw

Moderator
When a thermostat goes bad on a GM vehicle, the PCM sets a P0128 trouble code and commands the cooling fans to run and disables the temperature gauge

This doesn't make sense for our platform, or your situation. If your operating temp is too low, by 20 degrees, why would you need the fan clutch to engage and cool it further?

I agree, you do need a new thermostat, I just also think you have a secondary problem, that's either your sensor or the stepper motor, depending on what your live data stream says when the needle zeroes out.
 

Mooseman

Moderator
I agree, you do need a new thermostat, I just also think you have a secondary problem, that's either your sensor or the stepper motor, depending on what your live data stream says when the needle zeroes out.

:iagree:

A flaky sensor or its wiring could be sending bad readings and giving false readings. The sensor sends the info to the PCM and the PCM tells the cluster what to display on the gauge. That would also explain why it's setting the P0128 code. A bad stepper motor wouldn't do that.
 
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MRRSM

Lifetime VIP Supporter
Dumb Questions, perhaps ...But you haven't mentioned Checking the Coolant Levels in the Radiator (perhaps via a Pin-Hole Leak in the coils or leaking along the Plastic Side Seams?) and the Coolant Reservoir with the Vehicle being cool to the touch. Be curious to know is you are losing any fluid either to failing Water Heater Hose Connections at the Engine Block or Firewall.

The last place that comes to mind is losing any Coolant leaking from the Head Gasket and perhaps weeping the stuff away somewhere along the parting line on the Engine Block. If the Coolant level drops at or below the Engine Temperature Thermistor Sensor or below the Bi-Metal Brass Button and Steel Coil Spring inside the body of the Thermostat... their responses and reporting might change.
 
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OP
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vipergg

Well-Known Member
It was the thermostat . Replaced the stat and had them flush the coolant and reset the codes . Coolant levels were fine before all this . This has fixed the issue . Gauge is working again and reading right up at the middle mark on the guage, heat is great . I have taken it for 3 longer drives since and gauge is still working and I even stuck my scanner back on it and the ect is reading at about 198 to about 202 degrees where it should be . I would keep this in mind if you ever see this . I have seen other reports of this happening to where it disables the gauge when a p0128 comes up . Dont know why , but you can sit there and see the p0128 code on the scanner and as soon as you clear that code with the scanner the gauge works . Maybe they changed the behavior when they went back to a standard viscous fan clutch in the later years , I dont know just speculating . I found that info on the link I posted, the guy is a ASe mechanic . It certainly worked in my case .
 
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