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You have a vacuum leak somewhere. Either check all the vacuum lines or the intake tubing.

 

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hmmm........ ignition module causing th engine to go lean ? .... I am not conspiracy minded but if i was i would think they sold you a control module then found the leak

 

BTW ask for your ignition module back .. and then swap it back in when you get home ..

 

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Interestingly enough I was trying to find an engine schematic to identify my 2.7T cylinders and this thread had come up on google. I'm getting the same 2 trouble codes (cyl#4misfire, rndm misfire) but I am also getting a cyl#1 misfire. mine is an 01 A6 but with a manual transmaission instead of tiptronic, she's also got twice the mileage. Anyways I am glad to have come across this thread as I was getting ready to order replacement coils. I'm almost certain it's not the coils as my mechanic buddy had recommended(since they are generally very low priced from a salvage yard $15-$30). But it just seems too coincedental that both 1 and 4 are misfiring and always both or not at all. Now it's not misfiring when the engine is cold, the problem comes and goes, sometimes within minutes and on 1 occasion it did it for maybe the 1st 100 miles of a recent 1200 mile trip and then not again for maybe 3 days with about 70 miles per day. the problem is only during Idle, maintained speed, and light acceleration maybe 30% pedal or less, if I go any heavier on the accelerator the problem vanishes until I reach my desired speed, then it's right back again. Would you say this is what your A6 was doing? Did the problem go away with moderate-heavy acceleration? and then resurface once you level out? werll thanks for posting your problem and fix either way,

 

One of the problems with the shop, they like to install new parts, they never offer to get it rebuilt since it can take a week or so to get done. You can get the module rebuilt for much less than a new one. There are lots of companies out there happy to do this work.

 

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We(to humor me) swapped cyl 1 and 4 coils with 2 and 5 (1<->2 & 4<->5) yesterday. reading misfires in 1&4 still also getting a cat efficiency on warmup bank1(EC16815)& primary cat bank1 insuff effective(EC16805) however I beleive these 2 new codes are related to another issue.

So. I picked up a used module (Bosch 4A0 905 351 A) today $75 with 91 day waranty. wish I had the money to fix it with new parts but potential $500+ DIY fix they are about $250 from the local Audi dealer and probably needing 1 per bank hurts... I haven't checked anything aftermarket yet.

I should get around to swapping these modules around tonight and will update you with the results when I get them.