Facelift Coupe (UK - 55)
Last year (Dec 2011) my e46 developed the dreaded "random" alarm problem. Took it a local mechanic who replaced the siren saying that it looked rusty and was very likely faulty. The bonnet/hood switch was also replaced. Cost me a total of about £200 (£110 for the siren).
Last weekend (Dec 2012) I parked out side a video shop, chose a few movies and came back to a screaming car. Been using the key-in-door to lock the car since before tackling the issue myself and here's how:
(Note there are a lot of forums describing how to remove each of the sensors described so I won't go into those details.)
• Removed the bonnet/hood switch, shut the bonnet, armed the car, waited 30 seconds, ALARM went off .i.e. Not the hood switch
• Removed the tilt sensor in the boot/trunk (above the battery), armed the car, waited 30 seconds, ALARM went off .i.e. Not the hood switch
• Removed the interior motion sensor in the car, armed the car, waited 30 seconds - NO ALARM (possibly this unit, however the car had no sensors at that time either)
Took all sensors indoors, sprayed WD-40 on the bonnet/hood switch to clean it up a bit.
Didn't want to tamper with the tilt sensor. Shook it a bit listening for any loose pieces. No noises.
Removed the interior motion sensor from its casing and simply blew both sides of it to clean out any possible dust.
• Replaced the bonnet sensor, armed the car - NO ALARM.
• Replaced the tilt sensor in the boot, armed the car - NO ALARM.
• Replaced the interior motion sensor, the car - NO ALARM.
To make sure the alarm actually does works:
• Lowered the front windows, armed the car, waited 30 seconds, stuck my hand in through one window - ALARM!!!!
No parts required. Now that I've done it I would say it can be done in 45 mins.