Chart A-2 - No ALDL Data Or Won't Flash Code 12 (Check Engine Light On Steady)

There should always be a steady CHECK ENGINE light when ignition is on and engine stopped. 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 circuit from ECM terminal No. A5 or an open in diagnostic circuit No. C4.

1. If there is problem with ECM not allowing Scan tester to read serial data, ECM should not flash a Code 12. If Code 12 does flash, ensure Scan tester is working properly by testing it on another vehicle. If Scan tester is functioning properly and circuit No. C2 (serial data) is okay, the ECM may be at fault.
2. If light goes off when ECM connector is disconnected, then circuit No. A5 is not shorted to ground.
3. This checks for an open diagnostic circuit No. C4 (diagnostic request input).
4. At this point, CHECK ENGINE light wiring is okay. The problem is a faulty ECM. If Code 12 does not flash, ECM should be replaced.

Fig 1: Chart A-2 Schematic - No ALDL Data/No Code 12 (Check Engine Light On Steady)

Courtesy of ISUZU MOTOR CO.

Fig 2: Chart A-2 Flow Chart - No ALDL Data/No Code 12 (Check Engine Light On Steady)

Courtesy of ISUZU MOTOR CO.