Hi,
Hate to tell you, but you still have a MAF issue. This code - p1123 indicates that the ECU (DME in Porsche Parlance) has reached it's Lean Fuel Limit in response to a Rich A/F Mixture signal from the MAF but could not Lean it enough to achieve the proper mixture. You will most likely also get a second code - p1125 which indicates the same thing. The reason you didn't get it right away is that it had not completed one Drive Cycle. Once it did, and the condition persisted, it registered the fault and threw the CEL.
The MAF is a pretty sensitive instrument, it has to be. Personally, I would never go used, mileage in service means nothing. In this case, the used MAF isn't operating within Spec. Even if you don't throw a CEL, the DME can be running Fuel Maps which are less than ideal, but still within limits. This will cause less than optimal performance and MPG.
You could try cleaning it, a very delicate procedure, but I think you'd just be spinning your proverbial wheels. If you choose to try and salvage the used one, try the Mass Air Flow Sensor Cleaner from CRC Industries. Do not use any other type Solvents as they will permanently damage the MAF. The CRC Cleaner is about $6, see it at:
www.crcindustries.com. If this doesn't work, stop wasting your time and replace with a new unit. Hope this helps...
Happy Motoring!... Jim'99