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2005 gmc envoy_slt
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8 Posts
Discussion Starter #1
Hey all.
I'm having issues with my coolant temp sensor light. I've replaced the sensor, Thermostat and all new coolant. (the dealer wanted $260 to do a flush. I don't think so) It was fine for about 3 months now the coolant temp light and check engine light come on intermittently then go away for a while. Any ideas what's going on. When the coolant light comes one the the gauge also drops and rpm jumps thinking the engine is cold.
I have read the older posts about this but seemed the thermostat or sensor fixed the problems others were having.
 

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2004 gmc envoy_slt
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2,370 Posts
Sounds like you might have a wiring issue between the temp sensor and the PCM. Try moving the wire harness starting at the sensor and work your way back to the PCM as far as you can, Have someone in the truck monitoring the gauge while the engine is running.
 

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2005 gmc envoy_slt
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8 Posts
Discussion Starter #3
Thanks Envoyboy2004.
I'm hoping that's the problem and not a defective sensor. I'll let you know what I find.
 

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2005 gmc envoy_slt
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8 Posts
Discussion Starter #4
Hey EnvoyBoy2004.
It's been a while. I'm still having the same problem with the coolant sensor (PO128 code) but seems to be worse in the hot weather. I did tinker with the wiring from the coolant sensor and the coolant light went out, RPM dropped and all was good for 2 days until I turned on the A/C. Within seconds the light came on the gauge dropped and pegged at 100. (coincidence?)The A/C seems to have triggered this a couple of times in the last week. I have a new sensor that I will replace this weekend. Is it possible to have air in the system that would cause this? I've always been able to diagnose and repair problems but this intermittent problem has me stumped.
 

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2008 gmc envoy_slt
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14 Posts
same problem , just had my sensor changed $170 and after a week the gage just drop to 0 ?? any answers ?
 

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2003 chevy trailblazer_ls
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472 Posts
Hey all.
I'm having issues with my coolant temp sensor light. I've replaced the sensor, Thermostat and all new coolant. (the dealer wanted $260 to do a flush. I don't think so) It was fine for about 3 months now the coolant temp light and check engine light come on intermittently then go away for a while. Any ideas what's going on. When the coolant light comes one the the gauge also drops and rpm jumps thinking the engine is cold.
I have read the older posts about this but seemed the thermostat or sensor fixed the problems others were having.
After you have driven it and it warmed up does the gauge show a cold or cool motor? Also, you may want to see what a scan tools shows actual temp at. My Trailblazer was doing that and turned to be a bad sensor. But you've replaced it already. Could have got a bad one though. I replaced mine again a few months ago because it was reading erratic but it failed in about a month. It was hard to start the engine and when the engine was stone cold the sensor data showed over 300 degrees. So if you can see what temp the scan tool reports when it's messing up.

Also, My thermostat went bad and caused the same code you have and the gauge to drop.
 

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2008 gmc envoy_slt
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14 Posts
Ok I just had the temp sensor replaced work for a week , again drop down , call my mechanic he said lets change the thermostat it must be staying open, bring it to the shop and its working again ?? no code, should I replace it any way ?? need some help, this is very frustrating
 
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