Over the few years I have had this car (BMW 318i e46 N42) I have come across a few issues which were down to wear and tear of sensors / consumable parts and also some faults. Below is the list of the problems I faced and how they were resolved.

Note: The remedy for each fault may 'not always' be as below.

PROBLEM	FAULT	REPAIR
Engine cuts outs randomley every now and then while driving (and red light goes on) and it is not possible to restart the engine for upto 20 mins, no matter what you do.	Crankshaft Sensor Faulty - Needs replacing.	This Sensor costs about £68 from BMW.
Every now and then the engine takes up to 40 seconds to start, even if you keep the key fully turned.	Cam Sensor Faulty - Needs replacing.	This Sensor costs about £68 from BMW.
You hear a loud crack while rolling up / down your electric window, after which the window may not move any more or only very slowly.	Window Regulator Glider clip has broken - Either DIY repair it, or replace the whole Window Regulator.	The Glider Clip is not availabe as a seperate part, the whole Window Regulator Costs about £85 from BMW.
Most times the Air Conditioning does not give cool air, however some times it does.	Either the Air Conditioning System needs regassing or Check if the Compressor is engaging, if not, then check if the Radiator fan is spinning. If the Radiator fan does not work, then replace the fan, it will fix the Air Conditioning The Air Conditioning System requires re-gassing every 2 / 3 years. It can be done at Kwik Fit, costs £45 and takes 1 hour. Private Air Conditioning companies charge £80.	The Radiator fan costs £280 from BMW and £138 for a non original one.
A bulb warning light indication keeps comming on, even though the front or back light bulbs are ok.	A particular bulb socket and contact is faulty - replace them and also housing.	Parts cost £35. BMW total cost £155.
The engine yellow lights goes on and the car engine knocks / vibrates. You need to take the car up to 4000 revs to be able to change the gear to stop the car knocking. Loss of power, i.e. you only get 75% of the engine power.	One of more of the Ignition Coils are faulty - Needs replacing.	Change and 4 Spark Plugs too when you change the Ignition Coils. Each Plug from BMW costs £8 and each Ignition Coil costs £35.
The Radiator Fan turns on automatically while the car engine is already switched off. It remains on until you do something about it.	The Radiator fan is faulty - Needs replacing.	The Radiator fan costs £280 from BMW and £138 for a non original one (GSF).
Air bag light goes on intermittently, may be only when the passenger is in the car	The occupancy sensor matt for passenger recognition is faulty. Needs replacing.	Parts cost £80. BMW replace the matt for £307