90 GM w/5.0 blows cold air instead of heat

[adamcarothers]

I've recently had a problem with my car blowing cold air instead of heat. It has climate control. I changed the thermostat and the heat worked for a little while, but now it's back to cold air. It has gone back and forth a couple of times. It doesn't matter where the controls are set. Please help, I'm freezing!

[BizzareOne]

More than likely its your HEATER CORE. It sits just inside the passenger compartment up against the firewall on the passenger side. Its a REAL PAIN to replace by yourself.

Take it to a mechanic that you trust.

[jimmytroanoke]

Before you go pulling the heater core check for flow. There are a lot of other things that could cause your heater to not blow hot. Assuming your antifreeze level is ok, with the car hot and running feel both heater hoses where they come out of the firewall. If they are both hot, you have flow. If one or both are cold, there is not any flow and the core may be clogged. If the blend door in the system is stuck, broken, or the actuator is not working, the blend door may be in the cold air position. Can you hear the blend door moving when you cycle the temp knob from cold to hot? also look under the dash when you do this and see if anything is moving. You may need a service manual to locate all the various components. Depending on whether your system uses vacuum or electric to move the blend door, various other concerns come into play such as a bad control panel in the dashboard, a vacuum leak, or even something as simple as a fuse! My point is, dont replace parts randomly without troublesooting. Diagnose the problem instead of randomly picking the most expensive repair. If the antifreeze has been serviced regularly in the car's life, the core will clog very rarely. They usually leak before they clog. Is your temp gage running in the normal range? If the engine thermostat is stuck open, your engine may not be warming up completely and your cooling air may be lukewarm at best. If you are not comfortable diagnosing the problem find a competent mechanic or dealer shop to find the problem.