So I had to replace my driver side cv axel and was always getting a p0128 error code (engine coolant low temp) so figured I would do the thermostat replacement because I thought it was stuck open.
Looking at getting to it I new it would be a pain, but I noticed a silver insulated wire hanging down. Its the female end of a 2 prong round plug and the male end is just below the thermostat on the right side if looking at it from the wheel.
i managed to get it back in , real tight to get a hand in there.

Two questions

1. Is this the engine coolant sensor ? When I google most videos show a plug on the passenger side that needs modified deep socket, maybe that's the engine temp sensor ?

2. Can it only go in one way or does it matter which of the two prongs go in the female end?

In the picturs its the silver colored wire with the orange plug at the end. I read sonething about if it's unplugged it defaults to like -40 temp and throws off fuel mixture etc. That seems a little extreme for a default setting, I don't get any other errors and engine temp seems ok.



2007 H3 Hummer
3.7L

 

Typically the Coolant Temperature Sensor is located on the passenger side very close to the firewall, and is fairly difficult to get to. I'm honestly not sure what that sensor is in your photo; have you owned the vehicle from new, or did you purchase it used from somebody else?

As for your P0128, I would think that replacing your thermostat would fix this problem. Does your coolant temperature needle on the dash work at all? If it does, your sensor is probably fine. If the sensor were to **** itself, or become disconnected, etc. you would most likely get an "AC OFF" message on the dash; if the vehicle cannot determine how hot it is, or if it gets too hot, it will shut the AC off as a precaution.

I just rebuilt the engine on my 2007 3.7L last summer; there is no sensor on my vehicle in the location that is in your pictures. My vehicle is a late model 2007; if yours is an earlier model (window controls are flat on the drivers door, not slanted at a 45 degree angle), then there could be a subtle different between the earlier 2007s and the later 2007s.

One of the experts on here can chime in; I'm unsure what that sensor is in your photo.

 

 

Bought secound hand, don't think I have a curved window control.
I see lots of engine temp for the passenger, but this is like an inch of the hose where the thermistat is. Tack gauge usually shows engine temp around 1/2 way, but I always pop a p0128 after clearing then driving.

 

Not sure what your wire(s) go to or which you are referring to, but if you have not already changed the T Stat in that truck, I would be highly confident that your P0128 is telling you it is not working properly.

Many of us replaced at least one. My 07 had the code in winter because it was stuck partially open and would not allow the temp to get up to full operating range quick enough. The PCM throws that code if full operating temp is not reached in some certain time, like 5 minutes or whatever. The gauge would work fine, it just stayed low for a good 30 + minutes until things heated up. When it is warmer outside, that may be harder to see happen.

Yes, getting that I5 T Stat replaced is a PITA, but you can do it through the fender well. I cannot remember if I removed the fender liner or not when I did.

Mine was 9 years old at the time. I posted some quasi useful info back then... Stinking I5 T Stat Replaced

 

Is the sensor number 709 in the diagram below? If it is that is the knock sensor.