New 2008 A4 Quattro 2.0 owner with a few questions

My 1st question is regarding the cooling system on the A4 I recently bought, when starting car for 1st time of the day I've noticed it seems to take a lil longer to warm up than it should tho so far it hasn't went passed the 50% line once so it's not overheating I know this much lol. The issue is tht once engines warm as soon as heat is turned on ,temps even set as low as 68-72°, the needle starts falling back to the Cold side the higher fan speed is set the quicker needle falls. While heat is switched on I also noticed that cars idling rough wit what feels like a slight vibration but both go away as soon as fan sleed is turned down to at least 2 bars if not completely off, I've read this is most likely caused by my thermostat being stuck somehow in the open position, is this a correct theory? What if anything else might cause this to happen, I've yet to try running heater while driving on freeway but I know while driving in town town temps still fall to Cold..

An unrelated question I have relates to reading dtc codes, while running errands other day check engine light came on, since I was next door to o'reillys I used there code reader just attempting to get any info on light I might have been able to, if I'm not mistaken tht type of reader will not show all audi codes nor specific info on the ones it can read correct?
I ask because would it b possible for a p2187(I think that was it exactly) to be shown as 1 thing on reader but when I googled that code for Audi's it came up something diff