DTC Structure

Like all digital signals, DTCs are sent to the scan tool as a series of 1s and 0s. Each DTC is made up of two data bytes which each consist of eight bits that can be set to 1 or 0. The data is decoded by the scan tool to display each set of four bits as a hexadecimal number (0 to F) in order to display the DTCs in the conventional format. For example, P0420 - Catalyst System Efficiency Below Threshold (Bank 1).

Byte 1		DTC Byte 2
0000	0100	0010	0000
P0	4	2	0

The table below shows how to decode the bits into hex digits.

Binary Bit Pattern	Hex Digit	Binary Bit Pattern	Hex Digit
0000	0	1000	8
0001	1	1001	9
0010	2	1010	A
0011	3	1011	B
0100	4	1100	C
0101	5	1101	D
0110	6	1110	E
0111	7	1111	F

The first four bits of a DTC do not convert directly into hex digits. The conversion into different types of DTCs (P, B, C and U) is defined by SAE J2012. This standard contains DTC definitions and formats.

Binary Bit Pattern	SAE DTC Type	Binary Bit Pattern	SAE DTC Type
0000	P0	1000	B0
0001	P1	1001	B1
0010	P2	1010	B2
0011	P3	1011	B3
0100	C0	1100	U0
0101	C1	1101	U1
0110	C2	1110	U2
0111	C3	1111	U3

ISO 14229 sends two additional bytes of information with each DTC, a failure type byte and a status byte.

DTC Byte 1		DTC Byte 2		Failure Type Byte		Status Byte
0000	0100	0010	0000	0000	0000	1111	0101
P0	4	2	0	0	0	F	9

All ISO 14229 DTCs are four bytes long instead of three or two bytes long. Additionally, the status byte for ISO 14229 DTCs is defined differently than the status byte for previous applications with three byte DTCs.