Diagnostic Circuit Check determines:

• If SERVICE ENGINE SOON light works.
• If ECM is operating and can recognize a fault.
• If any codes are stored.

After performing basic diagnostic procedures listed under PRELIMINARY INSPECTION & ADJUSTMENTS , BASIC FUEL SYSTEM CHECKS (GASOLINE)  and C-4, BASIC IGNITION SYSTEM CHECKS . use self-diagnostic system for determining computer-related problems. See Figure.

After performing necessary tests in diagnostic circuit check and if no codes are indicated and driveability problems still exist, see TESTS W/O CODES article and SCAN TESTER USAGE in G - TESTS W/ CODES GASOLINE article in this section.

1. Check SERVICE ENGINE SOON light operation. Turn ignition on with Scan tester not connected, ALDL test terminal not grounded and engine not running. SERVICE ENGINE SOON light should be on steady. If light illuminates and stays on steady, go to next step. If light does not illuminate, go to A-1, SERVICE ENGINE SOON LIGHT INOPERATIVE. If light flashes, go to step 3).
2. Grounding ALDL diagnostic test terminal "B" now should cause SERVICE ENGINE SOON light to flash a Code 12, followed by any codes stored in ECM memory. If light goes from bright to dim or if light remains on and does not flash Code 12, see A-2, SERVICE ENGINE SOON LIGHT ALWAYS ON OR WON'T FLASH CODE 12.
3. If light begins to flash as soon as ignition is turned on, check for a short to ground on diagnostic test terminal wire between ALDL diagnostic terminal "B" and ECM terminal. See Fig 1 - Fig 3 . If circuit is okay, replace ECM.

Fig 1: Fuel Injection Diagnostic Circuit Schematic (Except 4.3L Turbo & Models With 4L80-E Transmission)

Fig 2: Fuel Injection Diagnostic Circuit Schematic (4L80-E Trans.)

Courtesy of GENERAL MOTORS CORP.

Fig 3: Fuel Injection Diagnostic Circuit Schematic (4.3L Turbo PFI)

Courtesy of GENERAL MOTORS CORP.