Reading & Clearing Codes
READING CODES (EXCEPT CADILLAC)
Fig. 4: Example of a code 12 displayed on the check engine lamp
Since the inception of electronic engine management systems on General Motors vehicles, there has been a variety of connectors provided to the technician for retrieving Diagnostic Trouble Codes (DTC)s. Additionally, there have been a number of different names given to these connectors over the years; Assembly Line Communication Link (ALCL), Assembly Line Diagnostic Link (ALDL), Data Link Connector (DLC). Actually when the system was initially introduced to the 49 states in 1979, early 1980, there was no connector used at all. On these early vehicles there was a green spade terminal taped to the ECM harness and connected to the diagnostic enable line at the computer. When this terminal was grounded with the key ON, the system would flash any stored diagnostic trouble codes. The introduction of the ALOL was found to be a much more convenient way of retrieving fault codes. This connector was located underneath the instrument panel on most GM vehicles, however on some models it will not be found there. On early Corvettes the ALOL is located underneath the ashtray, it can be found in the glove compartment of some early FWD Oldsmobiles, and between the seats in the Pontiac Fiero. The connector was first introduced as a square connector with four terminals, then progressed to a flat five terminal connector, and finally to what is still used in 1993, a 12 terminal double row connector. To access stored Diagnostic Trouble Codes (DTC) from the square connector, turn the ignition ON
and identify the diagnostic enable terminal (usually a white wire with a black tracer) and ground it. The flat five terminal connector is identified from left to right as A, B, C, D, and E. There is a space between terminal D and E which permits a spade to be inserted for the purposes of diagnostics when the ignition key is ON. On this connector terminal D is the diagnostic enable line, and E is a ground. The 12 terminal double row connector has been continually expanded through the years as vehicles acquired more on-board electronic systems such as Anti-lock Brakes. Despite this the terminals used for engine code retrieval have remained the same. The 12 terminal connector is identified from right-to-left on the top row A-F, and on the bottom row from left-to-right, G-L. To access engine codes turn the ignition ON
and insert a jumper between terminals A and B. Terminal A is a ground, and terminal B is the diagnostic request line. Stored trouble codes can be read 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 the light will flash once, pause; then two quick flashes.
This reads as DTC 12 which indicates that the diagnostic system is working. This code will flash indefinitely if there are no stored trouble codes. If codes are stored in memory, Code 12 will flash three times before the next code appears. Codes are displayed in the next highest numerical sequence. For example, Code 13 would be displayed next if it was stored in memory and would read as follow: flash, pause, flash, flash, flash, long pause, repeat twice. This sequence will continue until all codes have been displayed, and then start all over again with Code 12.
CLEARING CODES (EXCEPT CADILLAC) Except Riviera, Toronado and Trofeo:
To clear any Diagnostic Trouble Codes (DTC's) from the PCM memory, either to determine if the malfunction will occur again or because repair has been completed, power feed must be disconnected for at least 30 seconds. Depending on how the vehicle is equipped, the system power feed can be disconnected at the positive battery terminal pigtail, the inline fuse holder that originates at the positive connection at the battery, or the ECM/PCM fuse in the fuse block. The negative battery terminal may be disconnected but other on-board memory data such as preset radio tuning will also be lost. To prevent system damage, the ignition switch must be in the OFF position when disconnecting or reconnecting power.
When using a Diagnostic Computer such as Tech 1, or equivalent scan tool to read the diagnostic trouble codes, clearing the codes is done in the same manner. On some systems, OTC's may be cleared through the Tech 1, or equivalent scan tool.
On Riviera, Toronado and Trofeo, clearing codes is part of the dashboard display menu or diagnostic routine. Because of the amount of electronic equipment on these vehicles, clearing codes by disconnecting the battery is not recommended. Riviera, Toronado and Trofeo (Non-CRT/DID Vehicles) — Using The On-Board Diagnostic Display System:
First turn the ignition to theON position. On Riviera depress the OFF and TEMP buttons on the ECCP at the same time and hold until all display segments light. This is known as the Segment Check. On Toronado and Trofeo follow the same procedure, however, depress the OFF and WARMER buttons on the ECOP instead. After diagnostics is entered, any OTC's stored in computer memory will be displayed. Codes may be stored for the PCM, BCM, PC or SIR systems. Following the display of OTC's, the first available system for testing will be displayed. For example, 'EC?' would be displayed on Riviera for EOM testing, while on Toronado and Trofeo the message 'ECM?' will appear. The message is more clear on these vehicles due to increased character space in the IPO display area.
- Depress the 'FAN UP' button on the ECCP until the message 'DATA EC?' appears on the display for Riviera, or 'ECM DATA?' is displayed on Toronado and Trofeo.
- Depress the 'FAN DOWN' button on the ECCP until the message 'CLR E CODE' appears on the display for Riviera, or 'ECM CLEAR CODES?' is displayed on Toronado and Trofeo.
- Depressing the 'FAN UP' button on the ECCP will result in the message 'E CODE CLR' or 'E NOT CLR' on Riviera, 'EOM CODES CLEAR' or 'ECM CODES NOT CLEAR' on Toronado and Trofeo. This message will appear for 3 seconds. After 3 seconds the display will automatically return to the next available test type for the selected system. It is a good idea to either cycle the ignition once or test drive the vehicle to ensure the code(s) do not reset. Toronado and Trofeo (CRT/DID Equipped) — Using The On-Board Diagnostic Display System:
First turn the ignition switch to theON position. Depress the 'OFF' hard key and 'WARM' soft key on the CRT/DID at the same time and hold until all display segments light. This is the 'Segment Check.' During diagnostic operation, all information will be displayed on the Driver Information Center (DIC) located in the Instrument Panel Cluster (IPC). Because of the limited space available single letter identifiers are often used for each of the major computer systems. These are: E for ECM, B for 6CM, I for IPC and R for SIR. After diagnostics is entered, any OTC's stored in computer memory will be displayed. Codes may be stored for the PCM, BCM, PC or SIR systems. Following the display of OTC's, the first available system for testing will be displayed. This will be displayed as 'ECM?'.
- Depress the 'YES' soft key until the display reads 'ECM DATA?'.
- Depress the 'NO' soft key until the display reads 'ECM CLEAR CODES?'.
- Depressing the 'YES' soft key will result in either the message 'ECM CODES CLEAR' or 'ECM CODES NOT CLEAR' being displayed, indicating whether or not the codes were successfully cleared. This message will appear for 3 seconds. After 3 seconds the display will automatically return to the next available test type for the selected system. It is a good idea to either cycle the ignition once or test drive the vehicle to ensure the code(s) do not reset.