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HOMESERVICE MANUALSCHEVROLET1993ASTRO VAN PASSENGER EXTENDED, 4.3 ZREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROLS - BASIC TESTINGDIAGNOSTIC CIRCUIT CHECK (GASOLINE)A-2, MALFUNCTION INDICATOR LIGHT (MIL) ALWAYS ON OR WON'T FLASH CODE 12
1993 Chevrolet Astro Van Passenger Extended, 4.3 Z

A-2, Malfunction Indicator Light (Mil) Always On Or Won't Flash Code 12

1993 Chevrolet Astro Van Passenger Extended, 4.3 ZSECTION A-2, Malfunction Indicator Light (Mil) Always On Or Won't Flash Code 12
  1. Turn ignition on, engine off. If MIL does not illuminate, see A-1, MALFUNCTION INDICATOR LIGHT (MIL) INOPERATIVE. If MIL is on, ground DLC test terminals "A" and "B". Code 12 should flash. If Code 12 does not flash, go to step 2). If Code 12 now flashes, problem is a no serial data. Check serial data circuit for open or short to ground, or short to voltage between ECM and DLC terminal. If circuits are okay, replace faulty ECM or PROM (MEM-CAL).
  2. Turn ignition off. Disconnect ECM connectors. Turn ignition on. If MIL is on, check for short to ground in MIL driver circuit between MIL and ECM driver terminal. See Figure-Figure . Also, see WIRING DIAGRAMS article in this section in ENGINE PERFORMANCE for terminal identification.
  3. If light is off with ECM connectors disconnected, turn ignition off. Reconnect ECM connectors. Turn ignition on with engine off and diagnostic terminal not grounded. Using a test light connected to ground, backprobe ECM terminal circuit to DLC terminal "B". See Figure-Figure . If MIL does not flash Code 12, go to step 4). If MIL flashes Code 12, check for open in DLC test terminal "B" circuit to ECM. If circuit is okay, check for open in DLC terminal "A" circuit to ECM and ECM ground. See Figure-Figure .
  4. Check PROM (MEM-CAL) for proper installation. If installation is okay, replace ECM using original PROM (MEM-CAL). Recheck for Code 12. If Code 12 is not present, replace PROM (MEM-CAL). If Code 12 is present, system is okay.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.