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  1. #1

    Default 2004 GMC Envoy - SES light

    I have a 2004 GMC Envoy since 0 miles. Can someone please help me understand why the "Service Engine Soon" light comes on and then it goes off?
    I replaced the fuel cap, because that was the first problem I had years ago, but now the same light shows up.
    I went to the dealer and they found a P0128 code. According to them, I need to replace the thermostat for a $300 (because they need to remove the alternator).
    I don't believe that's the problem. Have somebody any advice, or have had a similar problem?
    I will appreciate any help...

  2. #2


    Your PCM monitors all your Powertrain inputs including your engine temperature. If the engine temperature is low after a predetermined length of time the PCM assumes the thermostat is stuck open. If the engine temperature is indeed low the most likely cause would be a defective thermostat. Another possibility would be a defective engine temperature sensor. Does your heater get hot immediately and stay hot or is it just lukewarm?

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    Welcome to the site!

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    Welcome to the site!


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    Welcome to the site!

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  7. #7
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  8. #8

    Default I having trouble filling the gas tank

    Also, besides SES came on (P0128), it takes 10 to 15 minutes to fill the gas tank because the nozzle continues to shut-off (like it is full) and I know it is not.
    I believe there is a Relation between P0128 and filling up gas tank

  9. #9


    I didn't try the heater yet...but please read my last post.. I'm also having problem filling the gas tank....

  10. #10


    I don't think the code is related to filling the tank. There is either some obstruction in your filler neck or the vent hose that goes from the top of the tank to the top of the filler neck is plugged up.

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