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Discussion Starter #1 (Edited)
I drove through some high water recently and the following day the alarm started acting funny. Within 5 or so seconds of arming the car, the siren goes off, without fail.

I replaced the bonnet (hood) sensor, the tilt sensor in the trunk, and disabled (unplugged) the ultrasonic (roof) motion sensor and the problem persists. Any recs?

Thanks!

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<b>SOLUTION: A failing car battery can cause the alarm siren to go off after arming the car. If you've tested the sensors, you probably just need to replace your car battery.

SOLUTION PART 2: Make sure your alarm fuse "Thiefproofing System" (fuse #49 and #67) in the glove box isn't corroded or blown as well.</b>

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Discussion Starter #2
I should mention that the car hadn't been driven in a while and the car has trouble starting up due to a failing battery. Could this be the issue? I understand there may also be a backup battery inside of the siren?
 

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'04 325i, '02 325iT
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I should mention that the car hadn't been driven in a while and the car has trouble starting up due to a failing battery. Could this be the issue? I understand there may also be a backup battery inside of the siren?
I see you mentioned swapping the switch under the hood, did you also try just unplugging it? It's possible your replacement switch is also bad.
 

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Discussion Starter #6
Just attempted to simply unplug the hood switch and sure enough, the car battery is completely dead. So, I suppose first things first, I have to change the battery. Would be great if that fixed the alarm upon doing so!
 

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Discussion Starter #7
Maybe your battery tray got wet or moist. IIRC the tilt sensor (if equipped) sits inside there. check it out too. maybe unplug it and see if the alarm still goes off.
I did replace the tilt sensor to no avail. Hoping a new battery works. Is there any reprogramming that needs to be done to a 325Ci if I change the battery myself? If so, I'll have to have it towed to a mechanic. Thanks!
 

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Swapping batteries is not all that difficult, just be careful when loosening the clamps on the terminals that you don't accidentally touch the other terminal with the end of your wrench or ratchet.

Keep the battery as level as you can when lifting in and out and don't drop it on your foot :thumbup:

I've had a few dead batteries before and it does seem to do some weird things to the alarm when it gets low. Usually the first thing that happens when I hook up a charger to the jumper posts under the hood is the alarm goes off. I'm assuming that is because the alarm was armed as the battery died, and as soon as it gets power it now sees the hood switch open.
 

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Discussion Starter #11
Thanks for the tips! Looks like I'll need a 10mm (terminals) and 13mm (mounting bracket) socket wrench and I should be good to go!
 

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Discussion Starter #12
I think replacing the battery fixed the alarm siren issue! However.... the car still won't fully start, shoot! It sounds like it's trying to fire up but never quite gets there. Fresh new battery. Maybe spark plugs need replacing?
 

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Discussion Starter #16
Used an OBD-II scanner for the first time ever - nifty tool! - and the errors were System Too Rich (Bank 2) and Trans Range Sensor Circuit Malfunction (PRNDL Input). Will begin my search for more info once I get home!
 

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Discussion Starter #18 (Edited)
FAULT: 00
PEND: 02

P0175 P - System Too Rich (Bank 2)
P0705 P - Trans Range Sensor Circuit Malfunction (PRNDL Input)

The descriptions came from the OBD-II manual. I think the second one is something I've dealt with for a long time that comes and goes, *but don't want to make any false assumptions*.
 
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