Where is the engine temperature sensor on 1999 firebird with 3800 engine?

Sounds like you probably have a bad MAF meter.

3800 has water temp sensor mounted on front of engine block kinda write below intake manifold and is plugged into block check there.

It's possible you got bad fuel or water in it as well.

service the engine.. change oil ,filter plugs etc

did you clear the faults before each read
Diagnose Buick 3800 Engine
unusual code that

P0125 is the OBD-II generic code for the engine computer, or Engine Control Module (ECM), not detecting enough heat in the cooling system before the fuel management system can take control. The ECM has a predetermined value after you start your vehicle to expect a certain coolant temperature

P0125 OBD II Trouble Code Insufficient Coolant Temperature for Closed Loop...

ecm runs the engine cooling fans

3800V6 engine over heating and heater produces cold air
try these

EGR vale not working
overheating
inlet manifold gasket leaking
water pump fins corroded
soft radiator hoses
try a new radiator cap

internal heating
heater control tap stuck
directional actuator flaps stuck

See if there is another fuse or relay under drivers side dash.

Hello smileynv: My name is Roger and I will answer for you.The 1988 Pontiac Firebird had a 2.8 v-6 engine. The timing set at 10 degrees in drive. The 1989 pontiac Firebird has both the 2.8 and a 3.8 engine.The 3.8 engine a DIS (direct ignition system) Had no distributor. The timing was preset and controlled by the computer system. There was no way to adjust the timing. Please rate the answer. This way I know it was useful. Should you need further help please just ask. Thank You for using Fix Ya. Roger

Possibly a weak fuel pump, idle air control valve, vacuum leaks, or loose or damaged wiring in the ignition system. Check for vacuum leaks first. That's the only one that won't cost anything. Make sure all intake bolts are tight, and all hoses are connected properly and in good condition.

Check to make sure the radiator fan is coming on watch it for ten minute as it runs it should come on if it doesn't replace the fan relay or the fan coolant temp senson located on engine block upper area of engine.

3 things to check, 1st, temp switch (controls turning fan on and off according to eng. temp, 2. cooling fan control, tells switch when to turn on and off, and 3. fan motor. Check it to make sure it isn't shot.

air filter dirty/bad engine ground connection/fuel not reaching injectors/loose connection on ignition system