Heater core? Very little heat

This should be an easy one. But I see a couple components that may have failed and am not sure how to test.
I have a 2006 tdi pkg#1, no climatronic. I can feel heat when the fan is off, but when I turn the fan on the heat disipates. If you let it go for a moment, it returns but goes away again after the fan is turned on.
Any ideas? Is there a heater core, and aux. heating element? I have acsess to a snap on modus- will it detect a trouble code? Anything else I should check?

How is your engine temp looking? If there are delays in reaching operating temp or the temp is exceeding normal operating temp I would look at a failed water pump/detached impeller.

But certainly no expert, not even sure the newer models haven't gone to a resistive element heater...

Oh, and how's your coolant level?

Engine temp hasn't moved from 190 since brand new. Coolant level is also
perfect. No signs of any leaks, be it oil, coolant or otherwise. The heated seats work, but it looks like they have a separate heating element under the front seats.
I'm unclear now how this system works. If I am understanding the Bentley manual there is a heater core and aux. heating element that work in conjunction with each other. I'm going to test the resistance on the aux. heating element tomorrow if I get a spare second.