P0721 means that the transmission output speed sensor is out of synch with vehicle speed. In other words, the trans is slipping. Could be something as simple as low fluid level, and the fact that the trans revved to 8000 rpm before slamming into gear might be indicative of low fluid. But it can also mean the electronic valve body in the trans is bad or trans itself. Obviously, checking fluid level would be the first step.

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'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
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