2000 Pontiac Grand Prix Gte - My low coolant light came on so I parked let it cool down went to auto store n got the Dexcool antifreeze and filled it past the hot marker. Drove it on the interstate and it's still not running correctly? Now when I shut it off my temp light comes on? Did I over fill & if so how do I correct this???
Over filling it by a little won't cause an issue. However you need to bleed the system if it was low. You could have a pocket of air stuck in the motor. You can youtube how to bleed your engine properly. If you bleed it out and it continues to overheat, have a shop do a block test. You will want that block test if you keep losing coolant and you don't have any coolant leaking under the vehicle.
To get that level of coolant back under the right level you can use a turkey baster to suck out the coolant till its low enough. The other way is to go under the vehicle when the engine is COLD, and open the drain on the radiator to lower the coolant level. You will then need to follow a topping off and bleeding procedure for your engines cooling system.
Because it overheated, you should replace the thermostat and radiator/reservoir cap.