The saga continues….
2009 R56 Non-S, 6-Spd manual, 187k miles

Recent repairs:
New timing chain
All new valves
All new valve stem seals
New water pump
New IN/EX VANOS solenoids
New oil filter housing
New thermostat/housing
New water pipe
New throttle body
New MAF sensor/housing
New PCV valve
New PCV hose
New IN/EX oil non return valves
New upstream O2 sensor
Professionally cleaned cylinder head
New head gasket
New valve cover gasket
New-ish cam position sensors

Current condition: P115C and P2187 codes after driving a few miles. No codes if left at idle in the driveway. Runs and drives decently, except for an occasional “stumble” while trying to maintain a constant speed. I also noticed that she hunts on cruise control which trips the P115C sooner than driving without cruise control. I’ve searched multiple forums for this code combo but haven’t had much luck with solutions. The P115C is pretty consistent and the P2187 only happens after driving for a good distance without clearing the P115C.

Question: Would the downstream O2 sensor trigger the above codes, or a bad CAT? I’ve set and reset the timing multiple times prior to finding a bad non-return valve, but I guess I shouldn’t rule that out. The only original parts are the MAP sensor on the intake manifold, the Valvetronic motor, fuel filter, and the downstream O2 sensor. Correct me if I'm wrong, but this code combo is curious. P115C "Mass Or Volume Air Flow 'A' Air Mass Too Low Compared To Model"(Rich) and P2187 "System Too Lean At Idle Bank 1"(Lean). It seems like I have a bad sensor somewhere, and seeing as how the MAP and downstream O2 are the only ones not new, I'm leaning toward the O2.

Any ideas? Something else I should look at?