blueskymine

Hi all,

I've got a 2004 Audi A4 3.0 Quattro, 6 speed manual trans with the BGN engine with 136,000 miles.

I've had an ongoing issue with the check engine light coming on for P0421 - warm up catalyst efficiecy below threshold, and in this case, the cat on the right side.

I have VCDS and have watched misfires occur on more than one cylinder (seems to be more with # 1, 2 and 3) while at idle, but not at an alarming rate, and presumably not enough to throw a CEL.

I went through the steps with VCDS for the readiness procedure and everything passed - which, among other things, says the oxygen sensor and cat are working correctly.

I have heard that a failing oxygen sensor could cause misfires and it could also be a camshaft position sensor.

I also read about a reflash of ECU after doing timing belt for camshaft position issues.

I replaced the timing belt, rollers, water pump, etc. at 111,000 miles as well as the spark plugs and the ignition coils were replaced about 4,000 miles ago at an Audi dealer paid for under an Audi emissions warranty type of thing.

It it possible the plugs I am using are incorrect (using part number PER6U11G) since it seems that the default engine code sometimes comes up as AVK.

Should I consider replacing an oxygen sensor on the right side, and if so, which one (pre or post cat).

Should I be concerned about the misfires or is it normal to have "some"? And could that be causing the P0421 code and/or ruining the cat?

Does the reflash of the ECU make sense and can I do that with VCDS?

I really don't think I need to replace the cat and obviously would like to avoid the expense.

Thanks for any input / advice / experiences.

Fred in South Jersey

blueskymine

Clarification and correction on the spark plugs that I installed:

NGK PFR6U11G

ct_esq

You need to replace the cat.

But it is the misfires that are sending unburnt fuel thru the emissions system and fouling the cat.

So, check all coil packs. The cam pos sensor can be a culprit as well.

rarely the O2 sensor is the reason. But when you change the cat, get new o2 sensors.

Just did mine. one cat, lots of $$. plus 4 o2 sensors. Car runs like a charm now.