i recently got a new thermostat placed in my car but it keeps overheating. what could be causing this?

Your check engine light "had gone off"? Or gone ON?? You can't use slippery, slangy talk like this, with mechanics!

First, check that your cooling system (radiator) is full.

Next, make sure that your fan works.

Is the radiator old?

Try one thing. Next time it starts to overheat, put your heat on hot, and your fan on high. First, is heat coming out, or cool? If heat, then keep it on. Does your temperature slowly drop?

Is it overheating when you are driving continuously, or only when you are going slow, or stopping?

Answer in the "Comment" section below.

Was the cooling system bled correctly? I know the 3800 engines are a ***** to bleed.

low coolant
bad waterpump
bad hoses
bad radiator
bad rad cap

Hi you need a new radiator cap.

http://www.engine-codes.com/P0128.html
http://www.obd-codes.com/p0128
im sorry but these could help you out.

Did you test the old thermostat on the stove in some boiling water? Then go through Pedros list.

Make sure the radiator fans come on when they're supposed to. Check the fan relay.

Did you put it in the right way lol

Plugged up radiator or a bad head gasket.

- Make sure there is a belt turning your water pump and that the belt is not slipping.
- Make sure the fins in your radiator are not plugged with debris.
-Make sure your radiator fans are working and that the shroud is in place.
-Make sure the radiator is full of water/ coolant mixture.
-Flush the engine block cooling system
-If you still having trouble, you'll have to pull the thermostat and make certain it is opening at the right temperature. You can put it in a pot of water and heat it up to make sure its opening at the correct temperature, which would normally be about 225 degrees F
-Make sure none of the radiator hoses are crimped which would throttle the flow of water through the cooling system.

water pump ?

The heat