From Auto data, general guide to fault codes

Failure type codes

All EOBD codes may have an additional two characters following the main trouble code which indicate the type of failure, e.g. P0351-XX, where 'XX' equals the failure type code.

Failure type codes are more commonly displayed using manufacturers' diagnostic equipment and can be used in conjunction with all EOBD code types, 'B', 'C', 'P' & 'U'.
Examples of failure type codes
EOBD code Failure type
00 No failure type information
01 General electrical failure
07 Mechanical failure
08 Data bus signal/message failure
0A General electrical failure 2
2A Signal stuck in range
3A Signal has too many pulses
49 Internal electronic failure
4A Incorrect component installed
A3 System voltage high
Example: Trouble code P0351A3

This code is logged when the system voltage detected is high in the primary or secondary circuit of ignition coil A.
P System - Powertrain
0 Code type - SAE defined (EOBD)
3 System identification - Ignition system or misfire
51 General malfunction - Ignition coil A, primary/secondary - circuit malfunction
A3 System voltage high
Hexadecimal codes

The EOBD trouble code is a hexadecimal number and therefore the third, fourth, fifth, sixth and seventh characters can be either a number (0-9), or a letter (A-F).

Unlike the decimal system that uses a base of 10, the hexadecimal system uses a base of 16. Using the characters 0-9 and A to F the number can be from 0-15.
Character 0 1 2 3 4 5 6 7 8 9 A B C D E F
Value 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

By using the hexadecimal system two characters can equate to a maximum number of 255. In the decimal system the maximum number with two characters is 99.

For example:
Hex Decimal equation Decimal
C8 (12 x 16) + (8 x 1) 200
FF (15 x 16) + (15 x 1) 255
100 (1 x 16 x 16) + (0 x 16) + (0 x 1)