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.
January, 6, 2012 AT 2:51 AM
Temp guage shows the engine gets up to temp. Neither heater hose has ball valve. Cables seem ok.
January, 6, 2012 AT 3:44 AM
The heater control is actually electric. I am providing you with the wiring diagram. Follow the left most fuse at the top and it will go down to the heater control temperature solenoid wchich has location on it. I believe it is on heater box. So, check the fuse, then the wires and ground for tight corrosion free fits and grounds should contact clean metal. Then I would suspect the control solenoid.
Before replacing it go get the trouble codes pulled. You can get this done for free at any Adavance Auto or Auto Zone.