heater/defrost warms up as engine does normally, but cools back down when vehicle slows or comes to a stop.
Nov 17, 2011 at 7:06 AM
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DRCRANKNWRENCH
✓Experienced Mechanic
3,380 POSTS
The heater works off of engine coolant which gets cooler when at idle.
The temperature is kept at operating temperture by the thermostat so it does not cool down too much at idle. More than likely you need to replace the thermostat which is probably stuck open.