How to change thermostat 2002chev trailblazer 4cylnder

Sounds like the sending unit.
That is where the wire plugs in for the temp gauge, on or near the thermostat housing.

need to jack up and remove drivers side tire and using long extensions unbolt the thermostat housing from the lower side of engine or remove a few thing [alternator belt etc] and go through top its tight but the following link has video of both ways of doing replacement
How to replace thermostat 4 2 envoy or trailblazer

For the I6 engine, the actual thermostat is on the drivers side below the alternator. The coolant temp sensor is in a bad place. Approx 06' and up, they changed the location to the drivers side, wayyyy in the back. PITA to get to. 05' and older had it located right above the thermostat, was easy.

What are you trying to replace?

Changed thermostat? Checked fan?

See picture below, thermostat is located on driver's side of engine. You will have to remove the alternator to get proper access to the thermostat.

The thermostat is located behind and slightly below the alternator on the drivers side. For the best access to the thermostat, you should unbolt and move the alternator to reach the upper bolt. The lower Thermostat bolt can be reached by taking off the front drivers side tire and reaching in with a 24" extension. The actual bolts are 10mm. Hope this helps...

Change the coolant temperature sensor. If the code is still there, change the thermostat, it might be stuck open causing the engine to run cooler than it should.

change thermostat

PO172 means that an oxygen sensor in bank 1 detected a rich condition (too little oxygen in the exhaust). I would replace the thermostat first. The computer could be injecting too much fuel because it thinks it never gets to operating temperature. That may keep the light off.