Having a blocked heater core will not cause the temperature gauge to register cold all the time, it will only prevent heat inside the car. If the gauge indicates hot at idle (after several minutes) but cools off as soon as you start moving above 30 mph then your thermostat is still stuck open.

 

Yea my brand new less then 6k thermostat is stuck open... though i have a bigger issue... i have the 12 valve 2.8 so to change mine yup you know it EVERYTHING has to come off.. FML

 

The in-cabin filter is on the passenger side, underneath the cover that hides the battery and heater core hoses.

You may have a problem with your blend door. It may not be opening correctly allowing heat into the cabin.

You could also have air in the system, too. Burping it wouldn't be a bad idea just to be certain.

 

Did you ever figure this one out?

I have a very similar issue with my 2.8 Q. At full HI heat, I get warm air.

Heater core was replaced by myself 3 years ago. It worked fine before and after changing the heater core but about 6 months after the replacement, it started not to heat very hot?

I'm thinking the controller or the vent flanges that direct the air through the heater core?

 

I have the same problem with my 1998 A4 1.8T. Does anyone know what the resolution is? Water temp guage behaves normally in the summer but now that it's getting cold it never moves and the heat is only somewhat warm. I was thinking maybe thermostat or water temp switch. I didn't see any codes but I'll check again.

 

heater core is probably clogged.