Question about GMC 2003 Envoy XL

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Check engine light on - 2003 gmc envoy XL

My check engine light (white light) has come on inside my 2003 gmc envoy XL? I took it to auto zone for the free reading and was given a code 0128. They think it maybe the thermostat or the coolant temperture sensor. The car is not over heating and it drives fine. Does anyone have any knowlegde on this problem or where the sensor maybe located? Thanks coolant level is fine and the code 128. thanks

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  • jmb4810 Jul 28, 2008

    I took my 2003 envoy truck to get checked for possible water leakage. I was told my theromastat was bad and needed replacing. What is the average cost and does the Envoy have a double thermostat?

  • beybgyrl2000 Sep 07, 2008

    i have the same problem. My light comes on for a week or so then it goes off and comes back on in approximately two days. I took it to auto zone and they told me the same thing about the themastat sensor. But having trouble with my mechanic finding it to replace it

  • Lizarrd Sep 10, 2008

    I had my 2003 Envoy checked out too and received the same code. I ordered the part through Pep Boys and after 3 weeks was told it was not in stock. Does anyone know where this part can be ordered?

  • tturtle Oct 26, 2008

    Trying to replace the tthermostat , I have a Haynes repair manual. It Tells you to remove the alternator then follow the upper radiator hose to the engine, this hose is nowhere near the alternator its on the opposite side of the engine. So where is the thermostat on the engine of a 2003 Envoy with a 6 cylinder 4.2 L engine

  • Anonymous Nov 09, 2008

    My service engine light is on, the truck runs fine. It would not pass inspection, I was told it ws the coolant temperature sensor. I picked one up t Advance auto parts but I cant find it on my truck. Anybody know where it is located? I have a 2002 GMC Sonoma 6 cylinder pickup. Thanksw.

  • ej_q29 Dec 07, 2008

    2003 GMC ENVOY 2WD 4.2 L Dianostic code 128
    Engine Coolant Temperature (ECT) sensor monitors the temp of the coolant.

    After several week of getting the "Service engine soon ligth" I decided to get a dianostic on my envoy resulted on ECT code 128.

    I will try to check the thermostat since my temperature gauge reads 168 F and is not moving at all and I think my thermostat is not regulating i'm assuming that is stuck open.

    my next try will be to replace the ECT sensor.

    Thanks for the previous people for the location of the thermostat.

  • Anonymous Dec 13, 2008

    I live in the city and only drive on the highway every so often, and thats when the light comes on. after a few days of coty driving it goes of again.

  • Anonymous Feb 06, 2009

    where is the coolant temp sensor

  • Anonymous Mar 20, 2014

    the service engine light is flashing on my 2003 envoy? what can it be?

  • Anonymous Mar 20, 2014

    where is the headlamp driver module located on a 2002 gmc envoy xl

  • Anonymous Mar 21, 2014

    Light service is on.Took to auto zone to check. They said it was a leak in the evaporator hoses

  • Anonymous Mar 25, 2014

    when I replace the camshaft sensor in my 02 envoy the old sensor was wet. I put the new one in and now the service engine light is flashing. what did I do

  • Anonymous Mar 30, 2014


  • Anonymous Apr 01, 2014

    Stalls when stopping at a light starts right back up runs fine


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That code represents a lower than expected reading for temperature (coolant). First thing to do is check your coolant level, as if it is low, then the sensor can not read correctly. Second, you may have a thermostat that is stuck open, allowing the coolant to cool off too much. The sensor should be mounted on top of the engine, near the thermostat housing.

Posted on May 18, 2008

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On 2004 gmc envoy's the thermostat sits behind the alternator if looking at the engine its on the right side. its hard to get to but if your patient and move a couple of wiring harnesses you can get to it thats also where you'll find the t-stat sensor but try the thermostat first if your temp isnt getting too high usually 210 is normal any higher then its over-heating and that could be the water pump or thermostat if its less than 210 then its staying open,

Posted on Oct 08, 2008

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The temp senser and the thermostat are located on the right side of the engine under the alternator. The alternator will need to be reoved for access.

Posted on Aug 23, 2008

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The temp sensor is located next to (forward) of the thermostat housing, which you can find by tracing the upper radiator hose back from the radiator to the top of the engine. The sensor will need a special tool for removal or a 15mm wrench if you are patient. When you replace it you will see why I say patient.

Posted on Jan 25, 2010

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2005 GMC ENVOY - Same problem as stated before however i had a loss of power when going up hill. Could this still be the same problem or is it something else????

Posted on May 25, 2010

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