2003 Audi A4, 1.8T quattro.
The heating doesn't work when the air speed in on high. Only cold air is blown out. When I change the air speed to very low, then it heats up a bit but not hot.

There are no errors on the dash or on the OBD2.
I replaced the Engine Coolant Temperature Sensor this week. The problem started after that.

 

How do the needles for engine and coolant temps respond to driving? Do they move: slow, fast, not at all?? Once you notice their response, you'll get a lot better advice.

 

mine started doing that too. i think its because ive been driving with a water pump leak for too long

 

Or thermostat jammed...

 

I figured out what was wrong in the end. When I changed the Engine Coolant temperature sensor, more coolant than I thought had leaked out. Once I topped up the coolant, it has come back to life.
In future, I think that is the first place I'd start.

 

how would i unjam it on an 04 a4?

 

there is no "unjam", you have to change it...

 

There are a lot of DIY's for the thermostat. It doesnt look too hard, I'm doing mine this weekend.