I need to change the thermastat ,,, where is it and whats the easiest way

Could be a few things. Blockage in the radiator, blown headgasket, bad fan. Need to check for those or get a shop to have a look.

IT MEANS THE ENGINE IS COLD FOR TO LONG SO ITS A TEMPERATURE SENSOR. YOU NEED TO CHANGE THE THERMASTAT IN YOUR CAR. THE GAGE IS WORKING OR YOU WOULD NOT GET A 1287 CODE SO DONT MESS WITH THE GAUGE, JUST GET A NEW THERMASTAT. AND UNHOOK YOUR NEGATIVE CABLE ON THE BATTERY BEFOR YOU REPLACE THIS AND TURN IGNITION TO START POSITION AND DO A TEN SECOND COUT, THEN REPLACE THE THERMASTAT , THEN HOOK YOUR NEGATIVE CABLE BACK UP THIS CLEARS THE CODES IN YOUR CAR SO IT WILL READ NEW CODES AS THINGS GO OUT. ALWAYS CLEAR YOUR CODES BEFOR DOING THE REPAIR AND DONT HOOK UP THE BATTERY UNTIL YOUR DONE WORKING ON IT,

You could try to change or empty the catalyst, if it's old and never changed, this could be the problem, as the exhaust won't let through easily enough. If i were you, i would empty it at first to make sure this is the problem, as well as getting about 15 horsepower more, probably more if the catalyst is very old. easiest way to empty it (at least with some other cars, i don't know about Pontiac) would be to take it off and use a crowbar to smash it up inside, and pour it out (yeah, i know it sounds suspicious, but it works)

make sure you have antifreeze in it start the car and see if its leaking out of the block...if so it soounds like a head gasket job ....if not it could be a water pump seal ....or a stuck close thermastat .....if your radiator is leaking from the front of the vehicle this would be the cheapest an easiest fix

Alot of time you have to bleed the cooling system after changing a thermostat it gets an air pocket in it look for a bleeder valve near or on the thermastat housing if so bleed it to get the air out.check the radiator and make sure its full because if you filled it up after changing the thermastat once the thermastat opened after the engine ran awhile well the cooling system needs refilled.

If the thermostat housing is leaking it could be a bad gasket, a cracked thermostat housing, or a crack in the intake manifold where its bolted on. Cheapest and easiest option would be to get a new gasket and a thin layer of blue permatex.

First make sure engine is cool.
-Put drain pan under car, coolant will run out.
-Remove the TOP radiator hose that runs to the metal thermastat housing. Coolant will now be running out, try and direct it to the drain pan below.
-Remove the thermastat housing, BEFORE removing the thermastat make sure you TAKE NOTE at how it is situated in the casting.
- Take the thermastat housing and run a wire brush on the flat spot were the old gasket material is.
-Both mating sides of the thermastat housing MUST BE PERFECTLY CLEAN. There can not be any old gasket material left.
-Insatll new thermastat.
-Put a very light amount of PERMATEX orange high temp. silicone on BOTH sides of the new gasket.
-Put a small amount on bolt threads also.
-Now install thermastat housing, and bolts.
-Tighten bolts down a little at a time, alternating between the two, unitl till they are tight.
-Install the radiator hose, and tighten hose clamp.
-Remove radiator cap, pour in coolant, until it sits just below the radiator cap neck. This may take a little time as the system has some air in it.
-Install the radiator cap. Make sure you radiator overflow tank is filled properly.
-Remove the drain pan, make SURE ALL the tools use used are secure, and out of the engine bay.
-Start engine, and place your heater control to 'HEAT"
-Let engine run until the top radiator hose is hot, which means you thermastat has opened.
-Keep watching the engine temp AT ALL TIMES, we wnat to make sure there is no air in the system.
-Inspect for any leaks. Now when the top radiator hose is hot make sure the hose clamp is tight.
Please let me know how it goes.

If you aren't loosing any antifreeze, you might check to see if the coolant fan on the radiator is coming on. If the thermastat is sticking your radiator hose will be solid to even swellen sometimes. Of course not to mention that if a radiator is over 5 years old, it could be bad, so you may need some things checked futher.

Changing thermostats really isn't a regular service but certainly needs to be done sometimes. The spring can get stiff. Try changing it first as that is the cheapest and easiest fix. If not that, there are several things it could be, first and formost though the water pump.

thers not need to take to garge