hope you checked power at the starter terminal 50 at the starter itself and also at the power junction in the K40 relay.

wont be the first to have that relay go on vacation at the worst time. if the key is able to turn in the ignition all the way to start and you also see the gear indicator show the correct gear position on the dash cluster - you are assured that the ECM is ready to fire the engine, but the starter is not getting any signal to do it or it has given up the ghost.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)