P1123 Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Lean Threshold
All the codes are here on my web site. Bookmark it:
Boxster Check Engine Light Codes
I was going to guess it was an O2 sensor. Be sure to replace them all at once (there are four on your car) or you may just confuse the crap out of your ECU with the sensitivity of the one vs. the dullness of the others. You can get less expensive replacements on
OxygenSensors.com that have the correct plugs ($110 or so per unit, but they have a cheaper brand...). Buy a O2 sensor wrench from Autozone and you can do it yourself and save money. You just need a jack to get under the back of the car.
http://www.oxygensensors.com/catalog.php?&pkey=1317203
I replaced all four of mine a while back and within a day of driving, you would have thought I got 10 hp more out of the motor. It's well worth the time and money to replace them. Fuel economy went up as well.