Heater blows out cold air half the time

sounds like a faulty sensor

1st check fuse to the heater
2nd check relay
3rd the filament wire of the heater

that happened to me the heater core went out ive read alot of different things about 03 galants most of them have the same problem it started with a grinding noise turning the temp contol, its actually the heater control motor value it causes the heater core to go out

Check out related help links with solutions to troubleshoot the problem:---
Click the link below:---
2004 Toyota Tundra: Oil Gauge and Temp Gauge Not Working?

http://howtobyme.blogspot.in/2012/05/2004-toyota-tundra-oil-gauge-and-temp.html

---------
Toyota 4Runner: Temperature Gauge Sensor http://technoanswers.blogspot.in/2012/04/toyota-4runner-temperature-gauge-sensor.html

-----------
Temperature gauge sits over half way mark? http://schematicsdiagram.blogspot.in/2012/04/temperature-gauge-sits-over-half-way.html
----------
Heater blows intermittently and temp gauge goes erratic?

http://technoanswers.blogspot.com/2012/01/heater-blows-intermittently-and-temp.html
------------- No heat on Oldsmobile Silhouette? http://technoanswers.blogspot.in/2012/02/no-heat-on-oldsmobile-silhouette.html
-------------
Temperature gauge does not move and no heat when heater is ON? http://technoanswers.blogspot.com/2012/01/temperature-gauge-does-not-move-and-no.html
----------
Temperature gauge keeps fluctuating up and down close to red? http://technoanswers.blogspot.com/2012/01/temperature-gauge-keeps-fluctuating-up.html
----------- Temperature gauge on the dash quit working on Pontiac Grand Prix? http://technoanswers.blogspot.com/2012/01/temperature-gauge-on-dash-quit-working.html
-----------
Dodge neon gauges not working? http://technoanswers.blogspot.in/2012/02/dodge-neon-gauges-not-working.html

---------
Heater blows cold? http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html

----------
Heat blows cold on drivers side and hot on passenger side? http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html

-----------
1992 Lexus LS400: Engine overheats, Antifreeze out of reservoir and heater Blows Cold Air? http://whatisbyme.blogspot.in/2012/05/1992-lexus-ls400-engine-overheats.html

-----------
These details will help.
Thanks.

replace your thermostat

first check to see that you coolant level is full, check it after the engine has cooled down,so there is no pressure in the system. otherwise you possibly have a head gasket in the early stage of failure

Definitely check your fluid level in your radiator. Top it off all the way. I've seen that happen multiple times on cars that were low on anti-freeze. If that doesn't work try replacing your thermostat. It's cheap and the easiest place to start if topping off the fluid doesn't get you fixed up.

water pump

sounds like heater core change it probably your reason.

the heater doesn,t work blows cold air