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A-2, Mil Always On Or Won't Flash Code 12

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.
  1. Turn ignition on, engine off. If MIL does not illuminate, see A-1, MALFUNCTION INDICATOR LIGHT (MIL) INOPERATIVE. If MIL is on, ground diagnostic terminal. Code 12 should flash. If Code 12 does not flash, go to next step. If Code 12 now flashes, problem is no serial data. Check serial data circuit for open or short to ground, or short to voltage between PCM and DLC terminal. If circuits are okay, replace faulty PCM or PROM (MEM-CAL).
  2. Turn ignition off. Disconnect PCM connectors. Turn ignition on. If MIL is on, check for short to ground in MIL driver circuit between MIL and PCM driver terminal (circuit 419). See Figure . Also, see appropriate WIRING DIAGRAM in the WIRING DIAGRAMS  section for terminal identification.
  3. If light is off with PCM connectors disconnected, turn ignition off. Reconnect PCM connectors. Turn ignition on with engine off and diagnostic terminal not grounded. Using a test light connected to ground, backprobe PCM terminal circuit to DLC terminals 4 and 5. See Figure . If test light does not turn on, repair open in DLC terminals 4 and 5.
  4. If test light is now on, check for open in DLC terminal 6 to PCM. See Figure . Check PROM (MEM-CAL) for proper installation. If PROM is installed correctly, replace PCM using original PROM (MEM-CAL) and recheck for DTC 12. If DTC 12 now appears, system is okay. If DTC 12 does not appear, replace PROM (MEM-CAL).
  5. Check PROM (MEM-CAL) for proper installation. If installation is okay, replace PCM 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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