Random Misfire

[flashjnr]

I'm new to posting questions and using vag com so please excuse me if I seem a bit of a Dumbo.

I have a 1997 4.2 A8. Very occassionally the cat light comes on when stopped at a junction. The faults are " random misfire" & cylinders 1,2,3,& 4 misfires. Checking forums found that normally this turns out to be either coil packs or lambda sensor. I swopped coils & plugs from bank 1 to bank 2 but fault stayed on bank 1. Sensor at fault? Where exactly is the sensor? Can I easily swop them to prove fault or is there a way to check them with vag com or meter, bearing in mind fault is very intermittant.

I would really appreciate help with this.

A very big thank you to everybody who contributes to this forum as I have already solved a few minor problems because other people have shared their experiences

[PsYcHe]

I'd check the ignition amplifiers first.. small black things on the front of the engine, swap them over and see if it moves the problem.

[ainarssems]

I doubt it is ignition amplifier as one feeds 1-4-6-7 and other 2-3-5-8 cylinders and usually they go down only for one at the time. Well You could try to switch them anyway just to confirm but I do not think that is the fault.

I would say it is either lamda ( oxygen ) sensor or cambelt timing is off for that side. Also could be clogged one injector on that side but that is a long shot.

[ainarssems]

For location of lambda sensor please see attached picture. There are 4 of them 2 on each side one before catalyst and one other after catlyst. If it is lamda sensor then more likely the one before catalyst number 6 in the picture the other one after catalyst is not shown in the picture but would be where the hole after catalyst is seen in the picture.

[PsYcHe]

Pretty sure that the banks are referred to as 1-2-3-4 and 5-6-7-8.

Swap the ignition amplifier across, should be a simple job and less hassle than a lambda.

[ainarssems]

Yes the banks are 1-2-3-4 and 5-6-7-8 but ignition amplifiers feed 2 coils on each side(bank) 1-4-6-7 and 2-3-5-8 and as I stated before they normally fail for one cylinder at the time. They normally fail due to cracked solder joints inside and with a lot of care and attention can be stripped of casing, resoldered and sealed with some silicon sealant. And I should know what I am talking about as I have managed to repair one out of three failed between my and friends A8 other 2 got damaged beyond repair wile trying to remove casing. All three have failed only for one output.