If your Audi A4 B6 alarm siren keeps going off by itself, the fix is almost always a leaking backup battery inside the siren module. Here’s the full Audi A4 B6 alarm siren repair, step by step, including how to open the module and replace the battery yourself.
This is episode 2 of our 2004 Audi A4 Avant Ultrasport project build. We kick things off with a look at a potential oil pressure issue we noticed, then move on to the main event: the factory alarm siren that had been triggering on its own. The culprit turned out to be a leaking 7.2v NiMH backup battery inside the siren module (part number 8L0951605A). These batteries degrade over time on B6-era VAG cars, and when they start leaking they corrode the board and cause all kinds of false-alarm behavior. The repair involves pulling the rear trim, cutting the module open, swapping the battery, cleaning the board, and resealing everything. While we had the back of the car apart, we also found a significant amount of pet hair and debris in the cargo area, and spotted a water leak that we’ll be addressing in a future video.
Table of Contents
Watch the Full Repair Video
The complete repair is documented in the video below. Use the chapter timestamps to jump to any specific step.
📍 Video Chapters
Tools & Parts You’ll Need
Here’s everything used in this repair. The 8L0951605A alarm module is a common part across a lot of B5/B6-era VAG vehicles, so used or refurbished units pop up on eBay fairly often if you’d rather replace the whole module than rebuild it. That said, soldering in a fresh battery is a fraction of the cost and keeps your original module in service.
OEM replacement module. Good option if the board is too corroded to repair. Amazon often has new or remanufactured units cheaper than the dealer.
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The B6 A4 rear trim uses Torx fasteners throughout. A multi-bit precision set covers everything you’ll run into.
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Makes pulling the old battery much cleaner than wick alone. Worth having for this kind of board work.
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Standard iron for soldering the replacement battery onto the board.
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Holds the module steady while you work. The module is an awkward shape to solder freehand.
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The exact battery type used in the 8L0951605A module. Make sure you match the voltage and connector orientation before soldering.
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Used to reseal the module after battery replacement. Creates a flexible, weather-resistant seal.
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Helps secure the module housing back together after resealing.
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Why the Siren Goes Off (The Battery Problem)
The alarm siren on the B6 A4 (and many other B5/B6-era VAG cars) has a self-contained backup battery. This is intentional: if someone cuts power to the siren before breaking in, the battery allows the alarm to keep sounding. The problem is that the original NiMH batteries in these modules age poorly. Once they start to fail, the module can no longer distinguish between a real power-cut and normal vehicle operation, so it triggers the siren on its own. In many cases the battery also physically leaks, which corrodes the PCB traces and connector contacts inside the module.
The part number on the B6 A4 Avant is 8L0951605A. This same module (or a close variant) was used across a wide range of VAG vehicles from the same era, so parts are relatively easy to find. You have two real options: buy a replacement module and swap the whole thing, or open the existing module and replace just the battery. The second option is significantly cheaper and is exactly what we do in the video.
If your alarm goes off randomly, especially after the car has been sitting, and there’s no obvious sensor trigger (no doors open, no hood ajar), suspect the backup battery first. A quick check is to pull the module and smell it: a leaking NiMH battery has a distinctive chemical odor, and you’ll often see brown or greenish residue on the board around the battery contacts.
Step-by-Step: Repairing the Alarm Siren Module
Disassemble the Rear Cargo Area
The alarm siren module on the B6 A4 Avant lives in the rear of the car, behind the cargo area trim. Start by removing the cargo floor board and any spare tire cover. Then pull back the left or right rear side panel (depending on trim level), which is held in place with push-pin clips and Torx screws. The module is connected to the car’s wiring harness with a single plug. Squeeze the tab, disconnect it, and the module comes out.
Be careful with the push-pin clips on the trim panels. They’re brittle on older cars and snap easily if you pry at the wrong angle. Work slowly from one end and lever straight outward rather than twisting.
Cut Open the Module
The 8L0951605A module is sealed from the factory, not screwed. You need to carefully score around the seam of the housing with a utility knife or thin cutting tool and pry the two halves apart. Go slowly and work your way around the perimeter rather than trying to pop it open all at once. You don’t need a perfectly clean cut since you’ll be resealing it with polyurethane sealant and tape.
Remove the Leaking Battery
Once the module is open you’ll find the PCB and the backup battery, which is a small 7.2V NiMH pack soldered to the board. In most cases on high-mileage cars, you’ll be able to see the electrolyte residue from the leaking cells right away. Use your de-soldering iron to heat each battery contact point and remove the old solder, then wiggle the battery free. Don’t force it if it’s still attached.
The leaked battery residue can be corrosive and may irritate skin. Wear nitrile gloves when handling the old battery, and dispose of it properly. Don’t try to recharge or reuse a visibly leaking NiMH cell.
Clean the PCB
Before soldering in the replacement battery, clean up any corrosion residue on the board. Use 90%+ isopropyl alcohol on a cotton swab or a soft brush and work carefully over the battery contacts and any affected traces. If there’s visible corrosion on the copper traces themselves, a fiberglass scratch pen or very fine brass brush can help restore conductivity. Let everything dry fully before moving on.
Check the connector pins at the edge of the board while you have it apart. Corrosion here can cause intermittent faults even after the battery is replaced. Clean those up with IPA and a brush as well.
Solder in the Replacement Battery and Reseal the Module
Drop in the new 7.2V NiMH battery pack, making sure the polarity matches the original. Apply a small amount of solder to each contact point, heating the pad and pin together for a second or two before touching the solder to the joint. You want a shiny, smooth connection, not a cold blob. Once the battery is secured, place the two halves of the housing back together, run a bead of polyurethane sealant around the seam, and wrap it with 3M tape to hold it while the sealant cures.
Reinstall the Module and Reassemble
Once the sealant has cured (check the product specs, usually a few hours), plug the module back into the harness connector and tuck it back into its location in the cargo area. Reattach the trim panels, push the clips back in, and replace any screws. Reverse the disassembly order and work methodically so nothing gets left loose.
Re-Sync the Key and Test
After reinstalling the module, the key fob may need to be re-synced to the car. The procedure is in the video and varies slightly depending on whether you have the original key. Lock and unlock the car a few times and verify the alarm arms and disarms cleanly, with no unexpected siren triggers. If it chirps normally on lock and stays quiet, the repair worked.
A commenter on the VWVortex thread for this video confirmed the battery replacement was still holding up a year after the repair. These batteries, when replaced with a good quality NiMH pack, should last a long time.
Frequently Asked Questions
Why does my Audi A4 B6 alarm keep going off by itself?
The most common cause on B6-era A4s is the internal backup battery in the alarm siren module. These 7.2V NiMH batteries degrade and start to leak over time, which causes the module to trigger false alarms. The fix is to open the module and replace the battery, or swap the whole module for a replacement unit (part 8L0951605A).
Can I just disconnect the alarm siren and call it done?
You can, and it will stop the false alarms. But you’ll also lose your functioning alarm system, which may affect insurance depending on where you live. Replacing the battery inside the module restores everything properly for much less hassle and cost than losing the alarm entirely.
What is the alarm module part number for the Audi A4 B6?
The part number is 8L0951605A. This module was used across several VAG platforms from the same era, so it’s a fairly common part. You can find replacement units on Amazon and eBay, often for significantly less than dealer pricing.
Do I need to re-sync the key fob after replacing or reinstalling the alarm module?
Yes, in most cases you will. After disconnecting and reconnecting the module, the car may not recognize the fob until it’s re-synced. The process is shown in the video at the 23:20 mark. It’s generally just a lock/unlock sequence with the key in the door, but watch the video walkthrough since it varies slightly depending on key type.
Is this the same module on the A4 B5 and other VAG cars?
The 8L0951605A (or the earlier 8L0951605 without the A suffix) was used across a fairly wide range of VAG cars from this period, including some A3, TT, and Golf variants. Check your specific car’s part number before ordering, but the repair procedure and battery type is essentially the same across all of them.
What else did you find during this repair?
With the back of the car fully disassembled, we found a significant amount of pet hair and dirt built up in the cargo area, which we’ll clean up in a future video. We also spotted a water leak that needs to be tracked down. It’s one of those jobs that always leads to more jobs.
Helpful Resources
📖 Audizine: B6 A4 alarm siren troubleshooting
🛒 eBay: search for 8L0951605A alarm module
🚗 USP Project Pt. 1: Introduction and initial inspection
💰 USP Project Costs breakdown
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