not heating properly audi a6 1,9tdi , 1999
 

My 1999 Audi A6 blows cold air even when the engine is hot. The only way that I can get any warm air in the cabin is to set both thermostats to Hi, have the defrost on front window and have the heater in econ mode. Even then it only blows moderately warm air and not enough to heat the cabin. Any ideas on what is causing this?

Flush the heater core. If it doesn't help, you may need to replace it. A cheap part but a lot of labour.

Do you have any coolant leaks? Air could be trapped in the system.
As GTA_Driver mentioned, system flush is the best option.

Also, try to drive up a very steep/inclined road and check/add coolant. Then drive Down the same and check/add coolant. I did this few times and got all the air out quickly. The best way really is to drive around, keep checking coolant level, open the reservoir cap (when cold) and add more coolant until level no longer drops.

The other possibility is that flaps that direct air through the heater core are not operating properly/stuck.

I had the same problem on my 02 A6. I used some red rubber hose to hook the heater core to the steam cleaner at work flushed both ways. I also filled the core with CLR and let it sit for 5-10 min the flushed with water. Refilled the cooling system and now I have heat.

Thank You for advice,will try .