CODE 12 - No Distributor Pulses
Trouble Code 12 is a special case code that is not logged into the ECM's Non-Volatile Memory (NVM) when detected. When the ignition is ON and the ECM detects no ignition reference pulses coming from the ignition module, it illuminates the Service Engine Soon lamp.
Code 12 is also used when the ECM is in Diagnostic mode (ALDL Terminal B grounded, key On, engine Off) to signal the beginning or end of a so-called 'diagnostic sequence'. This is when the ECM enunciates each stored code using the instrument panel Service Engine Soon lamp.
When in this mode, the ECM will begin by flashing code 12 (on the lamp) 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 have been displayed, the ECM will again flash code 12 to indicate the end of the diagnostic sequence. The cycle continues as long as the ECM is in Diagnostic Mode.
NOTE: The codes will not be displayed in order of occurrence but rather in numerical order.