GM 12 – ERROR CODE DIAGNOSTIC SYSTEM FUNCTIONING CORRECTLY
Code 12 is used on most systems when in the Diagnostic mode (ALDL Terminal B grounded, key-on/engine-off) to signal the beginning or end of a so-called ‘diagnostic sequence’, when the ECM annunciates each stored code in the ECM’s NVM on the dash CEL.
When in this mode, the ECM will begin by flashing code 12 on the CEL 3 times. This indicates the beginning of a diagnostic sequence. It will then flash any codes stored in NVM 3 times each before proceeding to the next code. When all stored codes are finished, the ECM will again flash code 12 to indicate the end of the diagnostic sequence. The cycle will continue as long as the ECM is in Diagnostic Mode.