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Mode Settings

Grounding terminal B of the ALDL connector by jumpering it to terminal A, with the ignition key turned to the ON position, engine NOT running, causes the ECM to enter the Diagnostic Mode. In the diagnostic mode, the ECM will display a code 12 by flashing the Check Engine/Service Engine Soon light (code 12 indicates the system is operational). A code 12 consists of one flash followed by a short pause, then two flashes in quick succession. This code will be flashed three times. If there are other trouble codes present, they will flash in a similar manner. Table 5 refers. If no other trouble codes are stored, Code 12 will continue to flash until the ground is removed from the Diagnostic terminal.

Trouble codes can only be obtained with the engine stopped. Grounding the Diagnostic terminal, with the engine running, puts the ECM into the Field Service Mode. In the Field Service Mode, the Check Engine/Service Engine Soon light will show whether the system is in open or closed-loop. In open-loop, the Check Engine/Service Engine Soon light flashes 2.5 times per second, in closed-loop it flashes once per second. The other diagnostic modes are most easily accessed by the connection of a SCAN tool to the ALDL connector, and by then placing the SCAN tool in the appropriate diagnostic mode, as directed by the instruction manual for the specific SCAN tool being used. For more information on SCAN tools, refer to  ALDL Connector. To remove any stored trouble codes and extinguish the Check Engine/Service Engine Soon light, it is merely necessary to remove the 20A Battery Fuse (  Battery Fuse 20A ), for a period of 30 seconds. After the fuse is replaced, the light will go out and any trouble codes will have been removed from the ECM's memory.

ALWAYS BE SURE that the ignition key is turned off whenever you remove any fuse in this system!

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