Why would the air conditioner work fine but the heater not work in a 1996-1999 Ford Taurus - Mercury Sable?

there are several things that it could be, the thermostat ($35), the heater core($150), or it could actually be the head gaskets ($110), or it can be a vacuum problem($30 Replace lines). the pricing is if you can do the work yourself, its parts only!

Another cause, especially with with ac working ok, is that the temperature blend door actuator / motor has failed.

This is a relatively common problem, and the repair is not too difficult. Autozone shows this part as only $15, and also provides an illustrated repair guide for the job.

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