I have a 2000 A4 quattro Avant with approx 118K mi. I have been getting the P1136, Long term fuel trim.....too lean code on and off for a couple of months. Engine is idling a little rough, MPG is down, sometimes a black carbon looking smoke in exhaust at idle.

I have had both the MAF and turbo valve replaced at the "advice" of the dealer. The code still was appearing until yesterday when I picked it up from the shop yet again and noticed that the light came on approx 5 seconds after startup, much quicker than it normally takes once reset, (the shop had reset it).
I put my scan tool on it today and I am no longer getting P1136 but 10 different codes all pertaining to O2 sensors, sec air sys, turbo valves, etc....

Could a faulty O2 sensor cause all these codes (see below)?

Also, in the process of looking for the "downstream" O2 sensor I could not find it beyond the cat. converter. The sensor or the wiring for this sensor are not installed. Should this car have a downstream sensor?

P1425 Tank Vent. Valve short to ground
P1117 O2 sensor heater circ, bank 1 sensor 2 short to ground
P1435 Sec. air inj sys pump relay circ short to ground
P1421 Sec air inj sys valve circ short to ground
P1289 Turbo bypass valve short to ground
P1473 EVAP Emis cont LDR circ, open circ
P0102 Mass or Volume Air Flow a Circ low
P1115 O2 sensor heater circ, bank 1 sensor 1 short to ground
P1114 Bank 1, sensor 2 Internal resistance too high
P0140 O2 sensor circ no activity detected, bank 1, sensor 2.