IBM 6150 Hardware Maintenance Manual page 41

TNL SN20-9843 (03-27-87) to 75X0152 or 75X0166
Two-Digit Display Codes During Diagnostic Programs
2-18 Service Manual
The following chart is for flashing (approximately one second) numbers
displayed in the two-digit display while running the standalone diagnostic
programs.
Number Description of the Problem
02 A read error occurred while using the diskette drive.
99
A program check or a machine check occurred.
c6 An undetermined programming error occurred.