Chart A-2 - No ALDL Data Or Will Not Flash Code 12 (Check Engine Light On Steady)

There should always be a steady CHECK ENGINE light when ignition is on and engine off. With diagnostic terminal grounded, light should flash a Code 12, followed by any trouble codes stored in memory. A steady light could be a short to ground in light driver circuit or an open in diagnostic circuit.

1. If there is a problem with ECM not allowing scan tester to read serial data, ECM should not flash Code 12. If Code 12 is flashed, ensure scan tester is working properly by testing it on another vehicle. If scan tester is functioning properly and serial data circuit is okay, the ECM may be at fault.
2. If light goes off when ECM connector is disconnected, then circuit is not shorted to ground.
3. This checks for an open diagnostic circuit (diagnostic request input).
4. At this point, CHECK ENGINE light wiring is okay. The problem is a faulty ECM. If Code 12 is not flashed, replace ECM.

Fig 1: Chart A-2 - Schematic

Courtesy of ISUZU MOTOR CO.

Fig 2: Chart A-2 - Diagnostic Flowchart

Courtesy of ISUZU MOTOR CO.