Boxster S CEL P1126 and high LTFT
 

Started off with 3 codes, P1126, P1130 and P1133. Cleared the codes and drove a bit and the 1130 did not come back. Added a full tank of high octane gasoline and a bottle of fuel system cleaner. Cleared codes, drove a bit. CEL came back with only the P1126 (Oxygen Sensing Adaptation, Lower Load Range, Bank 1) Took apart the MAF and cleaned it. Not experiencing any loss of power, acceleration, car runs smooth as could be when Idle. There is a very slight change in how it runs if it is started cold but that goes away after a few seconds.

Tried the oil cap test not sure what these results mean but when I take the cap off engine gets very loud and rough. Back to normal with cap on.

Long term fuel trims are stuck at 31.3 this doesn't change with at Idle or 2500rpm not sure if it will take the computer some time to catch up with this.

Is smoke test my next step of diagnosis? How can I test the O2 sensors before replacing them? I see on my scanner it provides the voltage the o2 sensors are running at.

You could swap the pre-cat O2 sensors from one bank to another and see if your code travels to the opposite bank, giving you 1124 and 1128 instead. My first guess would be that since you have the code on just one bank, it's more likely to be a sensor than a vac leak.

You may have to PB Blast/muscle some crusty sensors out, but at least you won't be paying a shop for a smoke test just yet. Reset your stored codes and trip odometer at this time, your codes should return around the 60mi/100km mark so you can diagnose it. If after switching them your fault is still in the same bank, consider an injector.

I'm in a similar situation, except I have 1124,1126, 1128 and 1130, meaning I have the same issue on both banks, so I'm going to have a smoke test done to fully diagnose possible vacuum leaks.