Zigsman,
Like you, I was first in disbelief that it was the heater core, I figured it would all go bad or non of it go bad. I also had non of the "normal" signs of the core. Coolant smell, misty windows, damp carpet etc....
And I have learned more since my original post, since bringing it to the dealer and pleading with them that it was anything but the heater core:
*there is no water valve that "normally" is the culprit on this version of the 3 series
*After BMW said it was the heater core, last week or so I didnt believe them (see my old DME Warning post were the dealer almost "F" me out of 2k, I don't believe it was intentional) so I started calling local Indys...I told them the "tests" the other indy shop and BMW ha run and they agreed it was the core
*BMW said that a backflush of the core had never solved the problem and that it would cost 200-some dollars...the Indy shops, work on Bimmers are not experts IMO, and they all said a flush would fix it.
*I called the dealership back and said to do the flush as I already owed them 60.00 for there time, they said because of either the fluid or material inside the core (I don't remember) that it was extremely unlikely a flush would solve it as calcium deposits "grow" in the core and cause partial clogs that can't be flushed.
*I was sick of being cold and know the cost for heater core was 2k, as thought, "what the hell.". After the flush the dealership felt bad for me that it didn't work and charged me 150.00.
*I noticed a slight improvement when I turned the knurl knob, as in the past it did little to nothing, other than that, passenger side still cold, my feet still on fire and my vents/deferost are luke warm.
OK NOW ON TO THE REPAIR:
*There are 2 brands/models of cores that were used denso and valeo. They are both around 100.00 but was told that u don't know which one u have until u remove it. (I think that is wrong as I pretty sure I recall therealoem showing the 2 cores based on production year/month.
*I am patiently waiting for a DIY as I have been getting more PM on this problem from other members since starting it last year...also every member I have talked to resolved the problem by replacing the core...some did thermostat/water pump only to end up having to replace the core in the end.
*After talking to the SA (not the mehicanic) their was no mention of reprogramming. I would think if u fully disconnected radio, there may be something to "code" but not mention was ever made of that...they just said, "The parts are cheap, it is an expensive fix since the entire dash has to come out."
THAT IS ALL I KNOW, I AM WAITING FOR SOMEONE WITH A BIGGER "SET" THAN ME TO TRY IT FIRST AND POST A DIY.
IMHO, I am sure if u r a skilled mehicanic u can do it...the one concerned u alread mentioned is the "paperweight" possibility...not sure how close u live to a dealer but after doing the install if a tow to the dealer and 100.00 "coding" bill is the only extra cost I, personally would take that chance (and save 1700.00 or so in labor).....I am more concerned about getting to the core.
Keep posted as I would be more than happy to attempt this project together (virtually) on some weekend with pics/videos to each other and working through it.
GOOD LUCK
A side note, when I had the care in the dealer pointed out a small oil leak at the oil filter base....wanted 610.00 to fix it (others on this forum were quoted 800.00). Dealer said 4-5 hours of labor and 20 dollars in parts....I bought the gasket and fixed it in 2 hours or so....point being I don't think it is as difficult as the price implies.
Ok that was a long message to type on my phone..my hands are cramping
GOOD LUCK!! (I am not gonna double check this whole message for spelling and grammar so please forgive any mistakes..)