October 30: LEMON manuals on a temporary server, it is slow and having issues, download the manuals you need! Later in November we will switch back to the fast server.

Diagnostic Circuit Check

The Diagnostic Circuit Check is a starting point to identifying a problem caused by the engine control system. The ECM will flash Code 12 with the "CHECK ENGINE" light. This indicates ECM diagnostics are working. Codes will start with the lowest code. If no codes were recorded, see TESTS W/O CODES article.

Fig 1: Diagnostic Circuit Check Flow Chart

Courtesy of GENERAL MOTORS CORP.