R56 won't start, only crank once
 

Car is 2007 R56 with automatic transmission. The past few weeks, it randomly had start issue, but usually the 2nd or 3rd tries worked. However this morning, the car didn't want to start at all, no CEL.

Not like the most starting issue, when I push start button, it tries to crank, but starter stopped immediately after it cranks. I am very sure it's not battery or starter. And it's not related to the cold weather, it happened last night when engine was warm.

The car ran very well before this happens, the only problem was rpm dips after a full stop. my thought is maybe the crankshaft position sensor is bad. Did anyone experience this before? or any idea what would affect it? Thanks

Found a video has the same issue as mine.

used BWM scan tool, clear all codes, restarted the car, same issue, checked the code again, 1 code popped up A0B2 terminal 30E/30L. seems it's wire connection issue. Anyone know what is terminal 30E/30L?

What makes you sure it's neither? how did you eliminate them from being faulty?

jumping the battery is quick and easy. if the car starts with a jump, you need a battery. if not then getting the starter out is also easy to fair task.

Thanks MiniToBe. problem solved. Tested battery voltage this morning, it's 11.75V. so I gave a jump start and it worked. I drove around to charge the battery and found the trunk is no fully closed. So the problem was the trunk light drained the battery overnight. But still curious why this happened couple times previously but engine will start after few tries.

at that time, perhaps the voltage was strong enough to push the starter. also warmer weather helps too.

glad it was that simple :)