I figured this is a long-shot to have diagnosed without taking it into a shop but here goes:

I had the K&N intake installed on my 1999 Carrera 4 (starter and oil pressure sending unit were also replaced) and it ran well for about a day but now has a rough idle on cold start and last time I took it out it stalled every time I came to a stop or took too long to shift without blipping the throttle. It seems to run well at higher RPM's. It's throwing the following codes: P0410, P1411, P1126, and P1133.

I replaced the MAF sensor with a new Bosch unit on a whim and cleared the codes but seemed to get no love for that. Research on similar CEL codes and behavior seem to suggest a vaccum leak or stuffed cat but I wanted to get some troubleshooting input before sending it to a shop. Any help is appreciated!

 

P0410 and P1411 I believe can arise from a bad SAI (secondary air injection) valve.

This link may be of some value:

http://p-car.com/996/diy/sai/mainpage.html

Since someone was just "at" the engine the valve may be ok but a hose may have been dislodged or an old/aged hose cracked or split due possibly to rough handling. It is kind of tight in there and patience and care is very important to avoid damaging any of the likely very old and fragile hoses. Even plastic parts, where the hoses fit, get brittle with age and even if the hose remains intact too much force can crack a hose fitting. Also, I'm not sure how close it is but it might be possible one of the hoses/fittings of the AOS was damaged and this could be the sourc eof the air leak.

P1126 - Factory Fault Code 356 - O2 sensing adaptation, lower load range, bank 1

P1133 - Factory Fault Code 358 - O2 sensing adaptation, lower load range, bank 2

These errors suggest an intake air leak. Not sure if a damaged hose could account for this or if possibly the intake system was removed to replace the starter and pressure sending unit and something wasn't put back together right.

This is assuming too there was nothing disturbed when the K&N setup was installed.

 

Thanks Macster... some good info to assist in the search. It definitely seems like it must be an air leak.

 

Page 531 of Porsche 996 The Essential Companion remarks that "The hot-film mass air flow sensor MAF is very sensitive to contamination, and is almost guaranteed to fail if a cone-style oiled air filter assembly is installed."

 

That is certainly a possibility but in the OP's case I'm thinking he'll find something was disturbed.

If so and he fixes it and the everthing is ok I would not be surprised to see him back after some time with a MAF problem (though K&N IIRC denies these claims of MAF problems with I believe the qualification of a properly oiled filter element) but if that happens that is a thread for another time.

 

I agree. However the complexity of the systems being designed make their tolerances highly critical. PDKs sending fault codes because of low battery power, car batteries going dead in just a couple of weeks from all the power drain built into the car, even when it is just standing there...etc. The days when K&N first came to market are very different from today. I'm not sure the new cars can tolerate aftermarket toys.