Phantom power drain and MOST optical bus issue

A6 All/ S6 All
#1

I’d been running a whole month without any CEL issues at all (!) until electrical gremlins began to pop up:

  • first, the backup cam died, but I reset an aftermarket optical+BT adapter and it came back to life
  • then, a phantom power drain left me with a dead battery after about a week of no driving, so I completely removed the aftermarket optical+BT adapter
  • finally, the MMI stopped working with an optical bus short circuit code.

I didn’t have VCDS so I couldn’t run the optical bus diagnostic. But I started to take out devices with a fiber loopback plug and (relatively) quickly discovered that the SiriusXM radio unit was the problem (and I’ve never had a Sirius subscription for the life of the car.) I took it out of the optical loop and voila, the MMI came back to life.

Does anyone think that my phantom power drain is solved?

Is anyone else experiencing an rise in electrical issues in these 14+ year old vehicles? Is there a place where we catalog common electrical problems?

#2

My sirius xm module was bad on my c6 s6 also and creating a most break so I removed it as well. It seemed to solve my problem. I replaced the battery on the S6 and now keep it on a tender everyday and drive it irregularly.

My '14 q7 battery just died also and I bought an oem and coded in the bem code. I’m guessing previous owner didnt do such causing premature damage to the 3 year old battery

Honestly I would like to be optimistic and say there is not a parasitic module killing these older first gen most cars. If I were you, always bring jump cables when u take an old audi out, that’s what I do…

#3

I always have jumpers. But I’ve tried to jump with a little econobox before—and we had to idle for 10-15m before I got enough juice to start. These cars need a lot of power to crank…

#4

The last time my car was draining itself (that I’m aware of), it was the fan motors in the radiator

#5

What is the connection between removing a device (sat radio) from the MOST bus and rough idle/misfires on cold start?

I swear the car was purring like a cat before my phantom power drain episode. The only other thing I did was clear all the codes caused by a battery failure/low voltage. Is there some sort of adaptation that has to occur before the engine purrs again?

#6

I was informed at one point that the battery health plays an important role on engine performance. Iirc this being a topic of discussion regarding a tuned c6 s6 platform, and I do not want to post misinformation however, maybe someone else can chime in.

I would consider replacing your battery and properly code it in. Have a feeling that may be a solution to your misfiring. Dont mark my words however, it’s just a test I would do based on your description of issues.

#7

This is true. Your MAF your coil packs 02s and injectors are driven off drivers. Some of those drivers are in the ECUs themselves. Depending on the health and ACTUAL battery voltage all of these are effected. Power is restricted in many cases where the battery voltage is low.

#8

This info should be highlighted.

I just put my battery on the CTEK for 10 days (AUTO + RECOND mode) until it reached (the final) stage 7. Now the engine idles better and the misfires on cold start seem to have vanished.

Prior to this, I had just completed an overnight charge on the CTEK (stage 5 or 6?), and while the engine started fine and the battery meter read 100%, I was still getting misfires.

S6 V10 Misfires on Cylinder 6 only on coldstarts and really random. DESPERATELY NEED HELP!