Q: Heater on 1996 Chevrolet Camaro

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The heater started blowing cold and then back to warm off and on during a 20 minute drive. Any ideas what it could be?
(2) Answers
Your problem could be air in the cooling system. Any air passing through the heater core will cause the heater to blow cold. When the engine is cool remove the radiator cap and check the coolant level. If it's low top it off and you will need to find and repair your coolant leak. If it is full there could be a problem with the air temperature door inside the heater box.
The heater core is likely clogged (or was when you wrote this question a long time ago). Unplug both hoses from the heater core and run water through each side, one at a time, to get all the junk out that will build up over time.