Assuming repairs have been performed properly and there are no DTC's; you have an electrical problem.
Blown head gasket?
You could have a bad water pump causing no coolant flow, inoperable radiator fans, a coolant leak,............
The thermostat.
The coolant is pumped from the thermostat housing into the top of the radiator but not until the thermostat opens.
The function of a car thermostat is to ensure that coolant gets to the radiator when it should. It blocks the coolant from reaching the radiator when the radiator is cold, and then lets it flow once it is heated up.
The function of a car thermostat is to ensure that coolant gets to the radiator when it should. It blocks the coolant from reaching the radiator when the radiator is cold, and then lets it flow once it is heated up.
the thermostat
The thermostat on a 99 Mitsubishi Galant is changed by draining the coolant, removing the upper radiator hose, and unbolting the housing. The thermostat can then be removed and a new one put into place.
By the thermostat
If it is leaking coolant. And you changed the thermostat. I would then move to this section. Flush the radiator and the heater core. This can be done at a certified shop with a reliable mechanic.
3.1 engine leaks at the intake gasket
If it's overheating, it cold be thermostat, radiator or possibly other problems. If it's not overheating it could be the coolant level sensor.