I had a P0444 code on my 2001 3.0i last summer. Replacing the purge valve [13-90-1-433-603 Fuel Tank Breather Valve (evap purge valve)] with a genuine BMW one ended up fixing it.

Prior to that, I mechanically and electrically tested it very thoroughly, with 12V directly, using the Foxwell (pulses it at ~1Hz for about 20 seconds), blowing through a hose to confirm the valve was actually sealing and then opening during all this, measured resistance at 27.8 Ohms. So it seemed perfect in every way, except was throwing the code.

Searching the intertubes, I found many many people who swore by replacing only with Genuine BMW, so that's what I did, and it solved it.

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2001 X5 3.0i, 203k miles, AT, owned since 2014