Reading Diagnostic Trouble Codes

Since the inception of electronic engine management systems on General Motors vehicles, there has been a variety of connectors provided for use by mechanics to retrieve Diagnostic Trouble Codes.
Also, the have been many names given to these connectors over the years and models.
Connectors
When the system was first introduced in 1979 (early 1980) there was no connector used at all. On these early vehicles there is a green spade terminal taped to the ECM harness and connected to the diagnostic enable line at the computer. When this terminal is grounded with the key on the system will flash any stored diagnostic trouble codes.
The introduction of the ALCL made it much more convenient to retrieve fault codes. On most vehicles this connector located beneath the instrument panel. However, this is not always the case. On the early Corvettes the ALCL is located underneath the ashtray. It can be found in the glove compartment on some early Oldsmobiles and between the seats on the Pontiac Fiero.
The connector was first introduced as a square connector with 4 terminals but became a flat five terminal connector, then elvolved into the 12 terminal double row connector that most of us recognize.
To access stored trouble codes from the square connector, turn the ignition ON and idnetify the diagnostic enable terminal (this will usually be a white wire with a black tracer) and ground it.
The flat 5 terminal connector is identified from left to right as A,B,C,D, and E. There is a space between terminals D & E to allow a spade to be inserted so trouble codes can be read when the ignition is ON. On this connector D is the diagnostic enable wire and E is the ground.
The 12 terminal double row connector has been continually expanded through the years as vehicles have gotten more on-board electronic systems. Despite this fact, the terminals used to retrieve trouble codes have remained the same. This connector's terminals are identified as A-F on the top row and G-L on the bottom row, reading from left to right, as before. To access engine codes turn the ignition ON and insert a jumper wire between terminals A and B. Terminal A is the ground and terminal B is the diagnostic request line.

Reading Codes
No matter which connector you have, the trouble codes are all read the same way, through the flashing of the CHECK ENGINE light or, on later vehicles the SERVICE ENGINE SOON lamp. Trouble codes are identified by the timed flash of the indicator light. When diagnostics are first entered this light will flash once, pause then flash twice quckly. This code reads as '12' which indicates that the diagnostic system is actually working. Code '12' will repeat continually if there are no actual trouble codes.
If there are touble codes stored in the memory, Code '12' will flash 3 times before the next code appears.
Codes are displayed in the next highest numerical sequence (ie; 13,14,15,etc).
Code '13' is read as flash, pause, flash, flash, flash.
Code '33' would be flash, flash, flash, pause, flash, flash, flash.
This sequence will continue until all codes have been displayed and then start over again, starting with code '12'.