Same mysterious problem!
I have a 2014 X3 (F25) and have the same exact issue. Back in 2019, the passenger, rear R & L seats lost heating function.
Issue:
The front drive works just fine button and heat wise The front passenger button lights up with the 3 stage LEDs but no heat and the ISTA+ registers the button presses. All 3 throw an error code with the LIN - Missing heating module and the recirculatory sensor AUC or something like that.
What I've done so far:
I checked the 4 20A fuses in the rear Fbox and the 5A one in the front Fbox and all is well there, there is also 14V at the power connector on each module. I purchased 2 used heating modules and tried to replace the front and rear ones with no luck, which leads me to think that the issue is somewhere on the LIN between the modules and JBE or chassis electronics (see attached image). That JBE module is attached to the fuse junction box behind the glovebox and it all makes sense until you realize that this module manages many different things like parking sensors, window motors, and so on.
If anyone out there has figured this fault our or knows how to test the yellow data wire without an oscilloscope, that would be a huge help.
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