While driving, my heater began blowing cold air. After stopping and restarting the car, warm air began to blow again, but it didn't seem to get too hot. The temp gauge was at its normal operating level. Stuck thermostat??
Heater blowing cold air on 2004 Chrysler PT Cruiser
A stuck thermostat would either stick open or closed and if the temp gauge is at normal that means the thermostat is working fine, I see a recirculation door in the heater box usually when these doors stick the heater will blow cold or hot depending in which direction the door is stuck in. About $15 for the part.
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You may just simply be low on coolant. I had the same problem on my cruiser, a little coolant in the radiator, problem solved.