if low voltage was the problem, there'd be more posts with your problem AND you'd likely have more codes. only ecu/voltage problems i've seen are from too much voltage-like 16v, not from too little.

how did you test the coil and find it's good? ohm tests will pass a bad coil. you need a spark tester.

any chance you have chinese parts on it, especially the new parts? oe for the speed sensor and i've never had a bad 1 out of the box. bosch only for cap and rotor, bosch or ngk copper plugs, no platinum. oe or bougicord wires only, not even bosch wires.

the 1 i had was the bolt from a previous speed sensor change was dropped into the bellhousing and laid there for months before getting spit somehow into the tone ring. if the screens are out, a rock could've kicked up into the bellhousing.

not saying my past is the only reason for 214, just the 1 time for me and i've replaced dozens of speed sensors. it COULD be a reaction to the flywheel bucking about while it's trying to run and NOT the real reason. a scope would be nice. had an 88 735 that kept throwing codes for the airflow meter. bmw issued a bulletin-if you have this code, check the speed senor on the ft. of the crank pulley. the mount gets loose and the sensor slides into the pulley, causing a hard start, poor idle and accel. the ecu couldn't decide what the airflow meter was doing and threw the code. the airflow meter door was swinging back and forth too fast and in a manner the ecu didn't like. the point is, the code might not be the cause, just a reaction.

start with the basics, check the timing belt with covers off, no crank pulley marks, they move with age. get a spark tester and be sure you 30-40kv available and start AT the coil, going to the cap, then the plugs. check fuel pressure and volume. gotta be there somewhere. good luck, chuck.