Antitheft Device Failure

[old son]

I have had a starting issue with my 2010 B9. I turn the ignition on and hear a beep. My display says 'antitheft device failure'. Is this a common fault and if so what is the remedy? Thanks

[Sol]

The most basic issue would be a damaged transponder chip in the key head. it could be more than that - as in a fault in the vehicle itself, on the coil surrounding the ignition barrel, the COMM2k unit under the steering wheel (It sends the transponder code to the BSI) or the BSI itself. But the key transponder would be the first port of call. Without it working, the car will never start.

The transponder is a small black oblong plastic-coated part in the key casing near the start of the blade, sitting in a little socket - they are robust but can be damaged by heat, water or mechanical impact (bending or crushing) - do you have another key to try? If so and it works, then the first key transponder is faulty. It needs no power so the battery for remote unlocking is not involved.

If you have another key and it does the same, you have a bigger problem with the other items above. If the spare key works then you need a new transponder chip, coded to the car by DiagBox/Lexia.

The transponder chip is this bit:

https://www.joom.com/en/products/5d4bd07...0101a7a383

[old son]

Thanks Zion, I will try my other key. The issue is intermittent, it works for a while then out of the blue I get a bleep and will not start. I'll let the forum know how I get on.

[cancunia]

If both keys give the same problem, it could be the transponder coil around the ignition switch as per Zion's post.

[old son]

I did have the transponder around the ignition switch changed about 3 years ago but it could be the same part faulty again.

[Sol]

Its a coil that receives the key transponder signal that lives on the outer ring of the ignition barrel. If it fails, no keys will work. The transponders are in the keys, the ring round the ignition barrel is the antenna for that system.

When the key is inside that coil, the coil powers the little black transponder chip in the key and it wakes up and transmits it's encrypted code - this is sent to the BSI by the COMM2000 unit under the steering wheel (same board the indicator stalk plugs into)

The BSI checks whether the key is valid, then when the engine ECU asks for authorisation to start, the BSI confirms. If no confirmation is received, start authorisation is refused and the engine won't start as the injection pump is locked out.

Unplugging an ECU has the same effect, it is instantly locked out from start authorisation. If anyone ever unplugs their ECU, the battery must be disconnected first, after waiting the statutory 3 minutes then have no battery connected while the ECU gets disconnected or reconnected and never ever put the battery on with just the BSI connected alone.

[old son]

Thanks for the detailed response. I've not tried my spare key yet but I will!!