Just incase anyone is interested, a quick how to, to replace the cat section on an A4 1.9Tdi AFN Engine code.
1. Put the front of your car up on ramps.
2. remove the air box, this allows you access to the 3 nuts holding the cat section to the turbo.
3. Remove the 3 nuts, 12mm ring spanner should do it. 2 are easy, the 3rd 'bottom one' takes a bit of time.
4. Get underneath the car, and remove the rear splash tray.
5. About the middle of the car you will see that the exhaust pipe has a joint, this needs to be removed. 16mm ring spanner, slide the coller back so the pipe can come away.
6. This bit is a bit fiddly. You need to remove the cat and section of pipe through the engine bay. As long as you ramps are high enough, you should be able to do this.
7. Once out, remove the 3 nuts/bolts from the other end of the cat.
8. Fit new/replacement cat to the pipe and install back into the car.
Hope this helps, my car now runs like a dream.
I had a look in the old cat and the inside had worn away to the size of a tennis ball. Luckly I replaced it before it got lodged in the pipe and ******** my engine!
You can if you want to save money and give yourself a few more BHP's, smash the inside of the cat apart and make sure you get all of it out. Put it back on and you should be ok.
The MOT test only measures smoke particles not Co2 etc...
Regards,
Jim.