Failure Mode Effects Management (FMEM), Code 998
WARNING: This page is about a different car, the 1993 Mazda Navajo. However, it is still accessible from the selected car via links, so may be relevant.
FMEM mode allows system operation when sensors fail or transmit signals which are out of normal operating range. When this happens, PCM substitutes a mid-range signal for defective sensors while continuing to monitor sensors. If faulty sensor's signals return to normal operating range, PCM will use those signals. A Code 998 will be displayed when FMEM mode is in effect.