GMC Envoy Thermostat Replacement

So, my wife comes home the other night and tells me her check engine light is on.  This is an absolute emergency in our house due to the fact that she will go out and purchase a new car if this is not taken care of in a timely fashion.  So I go out to the truck and put the code reader on it.  The error I get it P0128 stating that the coolant temperature is below threshold.  A quick bit of googling and I determine that this is probably due to the thermostat getting stuck open.  Since I have noticed lately that the truck was running a little cool, it seems a good first place to start.

Envoy P0128 Code

You will need some things to perform this replacement.

You will need a new thermostat, I recommend a Stant unit.


You will need some Anti-Freeze, I use Prestone concentrated and mix it 50/50 with distilled water.

To change this thermostat, you will need to drain the coolant from the radiator and motor.  To do this, you will make sure that car is cool, and then remove the radiator cap.

Remove This Cap

Remove This Cap

Once that is done, you will want to get some kind of container to catch the coolant in.  On the driver’s side of the vehicle, you will need to disconnect the lower radiator hose.  You can use a pair of channel locks or large pliers to get the clamp to “lock”.  You can then slide the lower radiator hose off of the radiator and attempt to catch all of the coolant.  You will drop some on the ground but you should be able to catch most of it.

Remove The Lower Radiator Hose

Remove The Lower Radiator Hose

Now for the hard part, the thermostat is behind the alternator.  It is also underneath the BCM bracket.  It was a complete pain to get to.  The next step to getting this out is to remove the serpentine belt.  Put a 3/8 inch socket in the tensioner pulley and release the tension, this is shown in this post.  It’s not the same truck but the engine is the same as well as the technique!  This will allow the belt to be removed.

Then you will want to remove the bracket that is above the alternator.  There are 3 15mm sockets that are holding the bracket on, these are marked in the picture.  There are also two smaller (I believe 10mm bolts) that are holding various clamps onto this bracket.

Envoy Top Engine Bracket

Removing the alternator consists of removing the 3 15mm bolts that are holding it on.  There are two on top of the alternator and one on the bottom.  You will also need to remove the electrical connections to the alternator to get it to come completely out.  I did not completely remove it, I just slid it forward enough to get to the thermostat.  Unfortunately I stopped taking pictures at this point due to the time constraints that I was under.  It was mother’s day and I needed to get to my mother in laws house.  Either way, I think I can describe the situation well enough to help anyone who is reading this.

After moving the alternator, i found that my hands could not quite reach the thermostat comfortably.  I ended up removing the BCM from the vehicle so that I could move the wires that were in my way.  Once removing the 4 10mm bolts that hold the BCM in I was able to remove the thermostat.

The thermostat is held in by 2 10mm bolts.  I removed the bolts and took out the thermostat and the lower radiator hose together as one piece.  I did this because it was too difficult for me to get the clamp off of the thermostat.  I had to maneuver the radiator hose out of the vehicle but I was able to get the whole unit out from underneath the vehicle.

Putting the new thermostat in is just the opposite of the procedure above.  When putting the thermostat back in, I mounted the thermostat first and then maneuvered the radiator hose into place and connected it to the thermostat.

If you are faced with this repair, I hope you found this article helpful.

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