If the rubber has deteriorated then you will still get hot air into the cabin. That heater control valve is a common failure point among BMW's from 94ish all the way up. I just replaced one on a 98 328i that would get ZERO heat into the cabin. Upon disassembly found it was clogged with the rubber seals that had crumbled. Cleaned all that out and got great heat into the cabin... lol at all times. Had to replace it so that the valve was able to open/close as designed.

Hopefully yours was just stuck? Fingers crossed for you.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)