Check Engine Light

The "Check Engine" light in my 1999 Ford Ranger 4.0L just came on - again. Its a steady light as opposed to flashing. It came on once before but after I momentarily disconnected the battery, it disappeared but I believe that whatever problem causing it to come on is still there. My OBDII Code device indicated a code PO171 and PO174 which were the codes related to "system adaptive fuel lean (Bank 1) and "system adaptive fuel lean (Bank 2) respectively. Does anyone know what "system adaptive" mean and what inputs to the PCM is/are causing the problem? Otherwise the vehicle drives OK. The PCM could be operating on default.

Don