I have a cheap cable from Gendan, not a proper Ross Tech one (puts on flame proof suit and asbestos helmet) and I have a aftermarket Blaupunkt HU in my Octy and I was having problems getting VAG COM to connect to any controllers. I have discovered that if I take the fuse out for the radio VAG COM works fine. When I installed the new HU I definately disconnected the K line (or whatever it is called), I know I did this as this as the aerial booster is plugged into that pin on the HU.

So why then does the radio mess up VAG COM. I am going to check the voltage coming out from the diagnostic port on the car just to make sure though.

The cable works fine on SWMBO's Leon.

I would like to buy a proper cable but I've been put on a four day week at work and cash is an issue, so no complaining about cheap cables!

Try searching here or at Ross-Tech for the aftermarket radio problem.

Could be that the radio's connection to the K lines is tying up the bus. You see many controllers in the car share the same bus. If one controller takes control of the bus and keeps it, no others can get on and communicate. Hard to say without more information what precisely is happening on your car.

I checked out the voltage between pins 4 and 7 with the radio and ignition on - just over 1V, so that doesn't look the source of the problem and I'm not going to fry my laptop.

What seemed to be happening was that I could test the connection OK and I could sometimes get into the instruments controller, nothing else worked and I got an error about unknown protocol 9C9C when I tried to scan the engine. Once I popped the fuse out for the radio (or radin as the handbook calls it) it worked fine. I only did this because last time I remember it working was before I changed the radio over.