HI there
First, since you put the factory crank sensor back in and the car started, you either have a bad connection or you do in fact need to get a crank sensor from the dealer.
The code is for the egr system. This shouldn't affect the starting of the car unless the egr valve is stuck open. If it is, then the car will have a difficult time starting.
P0401 MERCEDES-BENZ - Exhaust Gas Recirculation Flow Insufficient Detected Read more: https://www.autocodes.com/p0401_mercedes-benz.html
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