Check Engine Light
 

I was driving my 1999 Porsche 996 and the check engine light came on. No problem, the car was running fine. I took the car in and the technichian said the "lean limit was reached". On the service sheet it said P1123 oxygen senser 1-3 and P1125 oxygen sensor 4-6. The check engine light was cleared. Yesterday I was driving my car and again the check engine light came on and the car is running fine. What can be causing this????? Thank You.