I recently tore down a HP LaserJet CM4540 to fix a different error (59.00.F0) caused by a sticking solenoid. The repair for this is well-documented online, but when I reassembled the printer I got a new error: 99.05.56 Firmware Install Error. I knew this had to have been caused by an error in reassembly since I had done nothing to affect the firmware, and in a few minutes I had found it: a wide, flat ribbon cable on the far right edge of the DC Controller (J105) was left undone. Reconnecting this cable fixed the issue. There are 34 connectors plugged into the DC Controller (J101 and J102 are not used) so it is easy to miss one on reassembly!