jic777

Has anyone here ever experience this before? I have a 99.5 A4 1.8T Quattro. About a week ago when I again started using my heater, I realized it wasn't working properly. What happens is when I first turn on the heater; I get warm air like I'm supposed to. Then, about a minute or less later, the warm air begins to turn into cooler air. Finally, there will no longer be any form of heat coming out of all the vents even with the temp set to HIGH. I have tried to use both "auto/manual" settings & "fresh/recirculate air" and neither solve the issue. The A/C works fine though. Then, after shutting the heater off for about a half hour, I try to turn it on again and get the same results - heat temporarily and then cold air. What do you folks think? Thank you for your help.

Yippers®

heater lines keeping hot coolant from flowing thru the core.
Is the coolant temperature at the normal once the car is fully warmed? Are both the upper and lower radiator hoses hot once the car is fully warmed?

jic777

Coolant temp does reach the normal level when car is fully warmed up. Where do I find the radiator hoses you're speaking of? Does the A4 have vents behind the firewall that control air flow? Could those be inoperable? Thank you.

Yippers®

I rather doubt there is an issue with the fresh air intake vents as they are shared by the AC and you say that is fine.
If the coolant temperature gauge reaches normal, you get heat for a few minutes it sounds like the heater core might not be flowing properly. I'd suggest involving a trusted mechanic.