Diagnostic circuit check is an organized approach in identifying a problem created by an electronic engine system malfunction. It must be the starting point for any driveability complaint diagnosis, because it directs the service technician to the next logical step in diagnosing a complaint. Understanding the chart and using it correctly will reduce diagnostic time and prevent unnecessary replacement of good parts.

NOTE: Test numbers refer to test numbers on diagnostic chart.

Checks Malfunction Indicator Light (MIL) operation.
Checks if ECM's self-diagnostic mode is operating.
Checks if ECM's serial data output is operating.
Checks if engine will start.
Checks for any codes that are stored in ECM memory with engine running.
Checks for any codes that are stored in ECM memory with engine off. If no codes are stored and engine will not start, see appropriate CHART A-3 in BASIC TESTING article in this section.
Compares ECM's control data to typical data values.
Checks if codes are intermittent problems. If engine will not start and no codes set while cranking, see appropriate CHART A-3 in BASIC TESTING article in this section.

Fig 1: Diagnostic Circuit Check - Schematic

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.

Safety Warning

Diagnostic Circuit Check

Use the Manual With the Right Hardware

When to See a Mechanic