There are a few possibilities;
The thermostat is stuck open. If you have a temperature gauge you can eleimanate that one.
The heater core valve is stuck or the cable that actuates it is not working. Check for the heater hoses near middle of firewall. One will have a ball valve on it. Someitmes the clamp that holds it in place comes loose or it gets stuck. See if you can move it by hand, only when engione cool, and do so gently. Check under the dash and make sure that the cable from the heater control thatr actuates the temperature is connected and seated properly. Check other cables, especially the heater box door cable, to make sure they are connected and seated in clips.
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Friday, January 6th, 2012 AT 2:39 AM