I have a 2001 Honda Civic and sometimes when i turn on the heat it overheats and i just got a new thermostat and it didnt solve anything, when i turn it to the cold air, the motor cools down to regular temp., what is wrong with it?

 

It may be low on coolant. Park the car facing slightly uphill or at least on level ground (you want the radiator to be the highest point). Check in the radiator itself (not just the plastic bottle) after the engine is COLD. If it is not full to the top in the radiator, fill it up. Then leave the cap off turn the heater on high, start the engine and add more as necessary.

If you consistently run out of coolant in the radiator but have some in the plastic bottle, this could be just a bad radiator cap. Or it could be a leak in the engine (head gasket) allowing gasses to enter the cooling system and push the water out.