2003 Mitsubishi Galant Heater not working

put a radiator flush in your expansion bottle .run it for about 20 miles. then drain and refill with 50/50 mix of anti freeze that will clean both core of your radiator and the heater core of dirt and rust. then bleed the air out. by turning the heater on full .run the engine with the expansion bottle cap off till you start to feel hot water at the hoses .then refit the cap.

check radiator fluid is full the heater operates buy the radiator fluid flowing through the heater core. next your thermostat might be stuck in the open position thus your radiator fluid not heating up on cold days - no hot radiator fluid no heat. if fan is not coming on check fuse first if fuse is good check heater fan motor

just follow the top radiator hose to where it attaches to the engine. There should be a housing held to the engine by 3 bolts. On the v6 it's much easier to access if you remove the battery and the battery shelf before you start.

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water level in radiator full , and is blower working

This could be that the actuator that opens the door in the heater is dead.. or the actual heater-core is bad. Which is costly and expensive.

Check the fan itself for free turning--if it grinds or is hard to turn, replace the fan motor. The engine heat sensor reports temperature to the computer which turns on the relay and it turns on the fan motor. If you have A/C, turn it on--that should force the fan on all the time the car is running. If that works, then replace the sensor. If you don't have A/C, take a chance on the sensor--it's not too expensive. Wrap the threads with several turns of teflon tape when installing and don't over-tighten.

you are correct - its called a heater control module,mounted on heater case under dash - i believe it is only available at dealers,but i could be wrong

before u add water in ur radiator make sure u start ur engine and remove ur radiator cap and pour water to it when u see there is no more air in ur radiator u can close it and it will work good

ok here we go and you aren't going to believe it...
I replaced the thermostat step one

I noticed that I had a small leak at the engine side of upper hose fixed that by taking it off and cleaning it and putting the hose back on Step two

then while refilling the reservoir with coolant i found that the hose that was supposed to be in the fluid was curled up on top and not sticking down in the fluid where it should be.
played with the hose until i got it to fall down in the reservoir where it should be Step 3

I started the car with the radiator cap off and let it run until it got to normal operating temp. and keep filling the radiator with fluid until the level stayed constant. Then put the cap back on and test drove. Everything worked fine haven't had that much heat in awhile and it kept blowing hot even when i stopped and let it idle for ten min.during test drive. Checked reservoir when I got home and had to add a small amount of coolant but not much.

I really think that the hose in the reservoir not being in the right place and pulling air into the system instead of coolant is the real culprit of this drama, but i replaced the thermostat to be safe.