'Steady' CEL issue assumed to be SAI fault - PLEASE read & offer any advice (we're all stumped here in Stumptown).

Stock 1999 A6 AvantQ with 2.8 V6 gas engine, has 120,000 miles, always serviced per schedule, and usually filled with Chevron or Shell gas.

Car drives fine in general except one intermittent PITA issue that seems to trigger CEL...

When engine is nearly warmed up (say one mark short of mid-point on temp gauge), or fully warmed, and I've been driving but must stop for red light, the engine idles normally (around 850 rpm?) for minute or less, but then rpm suddenly drop a little (e.g. 600-700 rpm) for 2-4 seconds. It feels like a ‘rough idle’ with just a slight vibration evident as if engine was on verge of stalling. After 2-4 seconds of this 'rough idle' then CEL lights up and rpm returns to normal.

CEL is always 'steady' & never flashing. During this whole time my right foot is never on the gas (just brake pedal; sorry Toyota).

When this problem first started happening the CEL would stay lit for a few days but then go out without additional intervention. However, lately CEL stays lit for weeks at a time; I can feel the 'rough idle' happening even when the CEL is lit now.

Indie shop here (just Audi, Porsche & VW for 20+ years) says it's code P0411 for SAI system... But shop says SAI components (e.g. tubing, air pump & kombi valves) show no faults when tested separately.

Shop checked (using smoke) for vacuum leaks several times now. SAI pump looks & sounds fine. They've also compared exhaust gas readings against two other 2.8 A6Q in the shop & said my car was right in normal range. Shop’s even kept it overnight & hooked up car to computer while stone cold but didn’t get any more info.

So in frustration shop has done ‘induction system flush’, replaced relay (for SAI pump), checked both kombi valves & replaced one that seemed 'weak' (said flow was still acceptable range), replaced the MAF (no codes for it), and dumped in some seafoam chemical (made exhaust look like a shuttle liftoff). Drove around for couple more days & CEL came back in same manner as before.

I'm taking the car back later this week to have the second combi valve replaced (the one that tested 'normal' for flow) so when that's done the A6 will have two new combi valves.

While there, they will be replacing auxiliary fan too (seems to be on the fritz because coolant temp will start creeping past midpoint in stop/go traffic but returns to normal once at speed).

So if you've read this far, TIA, and I look forward to your thoughts on how to fix CEL so car will pass smog test.

 

90% of SAI problems are bad vacuum hoses - replace them!
9% are the combi valves and the Air pump. -
if you hear the pump run it is a bad combi valve.

The other 1% is caused by a little German guy that wants to mess with your mind...

 

Well, looks like replacing front O2 sensor finally did the trick. After previous work (bank 1 combi & actuator) computer was kicking out new codes (P0411 like before plus P1197 and P0154).

Shop couldn't reproduce idle problem after changing O2 sensor, & I drove around for several hours afterwards without any problems. Plan to keep flogging the car over the weekend.