That code is "unable to perform pressure switch calibration". Nothing you can do about it. The circuit board may be damaged by the power failure (brown out or surge). Need to call a experienced Lennox tech. Furnace may have a 10 yr warranty on parts and as long as it does not look fried/smoked they may get it under warranty.
Check the vent pipes outside for blockage B4 calling for service. That usually is the reason for that code.