Engine turn over wont start - 1999 BMW 323 Series

Assume you have gas. BMW does not like to start or run if the battery voltage gets too low (for example sitting in the driveway unused. It does not take long with all the electrical drain(computer, security, etc.). A peculiar item about M cars is if the oil gets 3 quarts low, the engine wont start to protect the engine. Other less common camshaft sensor, crankshaft sensor, ignition coil pack, fuel filter, fuel pump. If it does not crank but battery is fine, check the starter. Turn the ignition to ON but not START then if you cannot hear the fuel pump humming, the fuel pump itself or a relay to it is probably bad. Using a rubber mallet, give a couple of sharp blows to the underside of the fuel tank and then try to start. If it starts, fuel pump is bad. Fuel pump is located under the back seat on the passenger side-easy to get to.