The heater in my 1998 Chevy Malibu is working inconsitently. After the temperature gauge reaches its optimum temperature. I will turn on the heater and find that cold air is coming out of the vents. Turning the controller to increase the temperature does not help. I turn the heater off for a minute or two and turn it on again. This time heat is emitting from the vents. What may be the cause of this problem. Thank you.

Under your dashboard is a big plastic box which contains the heater, air conditioner and ducting for the defroster…Airflow through this maze is controlled by several movable doors whose position is controlled by servo motors, which are usually vacuum operated, but are sometimes electric…As you play with the heater controls, a control unit sends signals to set these doors to the proper position, depending on what you asked for. Repairs tend to be difficult and expensive as is all under the dash work…But you might get lucky… Remove the glove box completely, look inside the dash at the heater box and see if you can spot anything amiss. Some of the door controls might be external where you can see them…

Excellent post reply Mr Caddyman.

(No, it wasn’t me who gave you 5 stars)