Diagnostic Circuit Check Determines The Following:

WARNING: This page is about a different car, the 1995 GMC Sonoma and 1995 Chevrolet S10 Pickup. However, it is still accessible from the selected car via links, so may be relevant.

Whether Malfunction Indicator Light (MIL) works.
Whether ECM/PCM/VCM/VCM-A is operating and can recognize a fault.
Whether any codes are stored.

After performing basic diagnostic procedures listed under PRELIMINARY INSPECTION & ADJUSTMENTS; A-7, BASIC FUEL SYSTEM CHECKS, use self-diagnostic system for determining computer-related problems.

If codes are NOT indicated and driveability problems still exist after performing necessary tests in diagnostic circuit check, see TESTS W/O CODES - GASOLINE article and SCAN TESTER USAGE under SELF-DIAGNOSTIC SYSTEM in TESTS W/CODES - 2.2L article.

Check MIL operation. Turn ignition on with scan tester not connected, DLC terminal not grounded and engine not running. MIL should be on steadily. If light glows and stays on steadily, go to next step. If light does not glow, go to A-1, MALFUNCTION INDICATOR LIGHT (MIL) INOPERATIVE. If light flashes, go to step 3).
Ground DLC test terminal "B". MIL should flash Code 12, followed by any codes stored in PCM memory. If light goes from bright to dim or if light remains on and does not flash Code 12, see A-2, MALFUNCTION INDICATOR LIGHT (MIL) ALWAYS ON OR WON'T FLASH CODE 12.
Check for short to ground in diagnostic test terminal wire between DLC terminal "B" and PCM terminal. See Figure . If circuit is okay, replace PCM.