Hi guys, I recently purchased an audi a3 2006 2.0tfsi. I have a major drain on my battery, I drive the car roughly 60 miles every weekend and if the car is left for more than 2 days it will be dead, no power at all causing me to constantly jump it when needing it, I took it to a garage yesterday for a diagnostic which they then kept it in overnight as they struggled to find the drain.

When I eventually got my car back today I have been informed my ecu is faulty and causing the car to believe its still “awake” I have been quoted £2000 for repairs including new ecu, programming and also siren system as this was also drawing power. Doing research and also using chatgpt, it seems that it cant be the ecu for a few reasons, one being no codes at all. Has anyone had any similar problems, I have been informed the alternator is fine if this helps

Thanks in advance

First thing to do, since car is new to you is determine age of battery. If battery is old, say 5 years+, then replace it. If no paperwork, do some digging. Not sure on 2006 model if it needs coded to the car if you replace with VCDS? Most batteries these days are AGM. Fit based in same cold cranking amps. Again on batteries, there may be a date stamp on the terminals, or printed with serial number, etc.

You can get a cheap multimeter and charge the existing battery on trickle charge to do some tests. For arguments sake, say 12.4-12.6v from cold. Once running, the alternator should bring it up to around 14.4-14.7v.

I’ve never heard of a defective ECU causing battery drain. That said, if the ECU has been subject to water damage as it normally lives under the windscreen cowl, then anything is possible.

Car sirens have inbuilt lithium back up battery. They eventually fail. Search YouTube for fixes.

I’d also be looking at aftermarket electrical work, sub woofers, stereo, aftermarket lights, any additional security, etc.

If you or a friend has VCDS, run a scan. That might not tell you the source, but could point you to a potential culprit, as existing DTC errors are logged.

Any come back regards how you bought the car? You have some protection within first 30 days, unless private sale.

If you trust garage in their diagnosis, search out a business that can replace the ECU and recode. There are businesses such as Autotronics that do this type of work. Never done it myself, but worth investigating.

Edited May 19May 19 by spartacus 68

Thank you for your reply, I forgot to mention the battery was replaced in February of this year so brand new really, I did think that about the ecu Im going to take it to a vag specialist and see if I get the same answers.

Cheers