Question about 2002 GMC Sierra 1500

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Running cooler than normal (indicated by temp gauge), hot air not hot, thermostat not the problem (recently replaced), check engine light NOT on.

Temperature gauge shows I'm running cooler than normal. The hot air is only about 75% as jot as it should be. I have also had problems with the fan speed controls. It only works on the 4th and 5th highest speeds, and once turned on, it won't shut off. Even when the car is turned off, the ventilation fans continue to run. Not sure if this has anything to do with the truck running colder than normal. I have replaced the thermostat and topped off the coolant and it didn't change anything. I installed it correctly. Any ideas on what could be wrong?

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  • 87 Answers

As far as the temp of the air blowing...it in direct proportion to engine coolant temp...try a hotter thermostat....as for the faan running continuously that is a seperate problem and sounds like a faulty rheostat or fan switch...rheostat is elect, switch on the fan motor that enables different speeds and may beinternal on your truck. only parts store will know . if it is replace fan motor. if not , test switch and then test rheostat

Posted on Feb 09, 2014

Testimonial: "Thanks! The thermostat I used was the one recommended by autozone and was supposed to be compatible with my truck. They looked it up in their computer...."

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Posted on Jan 02, 2017

bobiii84
  • 65 Answers

SOURCE: i have a 2004 GMC sierra 4x4.heater and air

Well usually a thermostat is a cheap enough part that I would change it out just cause. Actually I just did that recently with my truck :) Also you can check and see if maybe the temperature sensor is bad and giving a false reading, but I would start with the thermostat if you haven't replaced it. You can always test it in water to make sure it's opening but most times it's easier to just replace. Good luck!

Posted on Jan 13, 2010

emissionwiz
  • 75197 Answers

SOURCE: '08 silverado. dash board diplays, engine running

P0128 OBD-II Trouble Code Technical Description

Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
What does that mean?

This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.
FB.init("dd7d9e9681341cde77587bc6a2029f6f"); OBD-Codes.com on Facebook Symptoms

You will likely not notice any drivability problems other than the MIL illumination.


Causes

A code P0128 may mean that one or more of the following has happened:

Possible Solutions

Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:

  • Verify coolant strength & level
  • Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
  • Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
  • Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
  • If the above items check out good, replace the thermostat


Posted on Oct 26, 2010

  • 1311 Answers

SOURCE: 2004 GMC Envoy.Check engine soon

This code indeed does indicate a bad thermostat, but not in a way as overheating but the coolant temperature is not rising fast enough at warm up because the stat is always in the open position.

The rushing air sound you heard was most likely the electronic fan clutch starting to go bad. This is a common problem.

Posted on Nov 24, 2010

bluebirdmate
  • 1190 Answers

SOURCE: after running for 20 minutes or more , the heater

Hello: Check the coolant level, fill up the reservoir and run truck, if reservoir shows low again fill it back up. If reservoir was low/empty the check for leaks. Do this before you replace thermostat. The heater core would not create this kind of condition.

Posted on Feb 09, 2011

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