What is difference between on an auto on thermostate

Replace thermostat Chevrolet Avalanche Ricks Free Auto Repair Advice

need to put your car on a scanner to properly diagnose your problems p0440 evap system,128 thermostate ,p0742 torque convertor,p0327 knock sensor. take your car to a good mechanic

sounds kind of unclear but my assumption is ure looking for the thermistor which basically makes ure cabin blower work at different speeds and base on that it should always be close to the blower itself by below the dash on the passenger side or near the middle under dash, and maybe even behind glovebox in rare instances, but if u meant thermostat then that should be on the engine compartment by the right side radiator hose assy if looking directly in front of ure car looking to the back.

Is it auto or manual. Does it seem to be using all the fluid from the plastic overflow/filler bottle after each start. If its a electric fan, does it work??

you can get a belt diagram from auto zone you may have a bad thermostate

1-your thermostate is stuck open half way. thermostate is a simple device that made of enlarging metals when heat is applied. it suposed to be in close position to allow engine heat up, when specified temp reached it opens and let it flow and radiator tries to keep coolant cool.when it is broken it stucks or it acts slow or it wont acts at all. in your situation it is stuck open half way. it allows enough water to be heated but flow is limited to heat cabin heater.

2-if there are cabin heater selenoid/selenoids for two different temperature for cabin left and right, on some instance those selenoid valves to adjust cabin heater stuck open or closed like thermostate acts on first case. so heat is not enough to heat up cabin heater.

you have something called a blend door they are common to go bad on theses vehicles they become stuck open causeing no hot air to get thru its inside your dash inside air vent

call the auto parts store and they will tell you

could be a bad thermostate or senisor take it to an advanced auto or to a autozone tell them you need a code check done theey will usely do it for free it will tell you what is going on gives you the codes the guys at the store can usualy tell you waht it is a bad sensor or thermostate

Take it to another shop, The water pump doen't hold the Thermostate. The thermostate is in the thermostate housing where the end of the radiator hose is connected to the egine. 
There are different thermostate rating to which temprature you'll like your engine running at. Most common would be a 190 thermostate you can get them from 160 up to 220. 
As for the water pump you would know if that failed do to over heating or leaking from the weep holes in it. 
Bring the truck to another shop and have them flush and Antifreeze if it's never benn done and change the Thermostate to a 180 or 190 and should be done every Two years do to waer and tear. Note the 180 runs cooler then the 190 and so on. 
I run mine on 160 since I tow a trailer and the radiator also cools the transmisson oil too. Just for though but you should be fine at 180 or 190.
Good luck and hope this helps.