what causes my heater to go cold after twenty miles?

Question

i can drive my car for twenty milesand the heater works fine. if i get off the highway or slow the car the heater goes cold. i went to work last night and the temp needle went all the way to the hot sidee. but after i started moving it came down to the normal heat range yet my heater still did not work.

Answer 1

The heater core could be plugged up. flush the heater core out.