Diesel Mechanic how-to

How to Read Heavy-Duty Fault Codes

Diagnostic software turns the truck’s data into clues.

Updated By the Career Education Institute Curriculum Team
Quick answer

Diagnostic software turns the truck’s data into clues. In short: connect; record; review, then complete the remaining steps below.

Step by step

  1. Connect the diagnostic adapter to the vehicle’s diagnostic port and launch the software.

  2. Record all active and inactive fault codes with SPN and FMI.

  3. Review freeze-frame or snapshot data and when faults occurred.

  4. Look up the codes in the manufacturer’s troubleshooting guide.

  5. Check related sensor values and perform guided tests.

  6. Repair the root cause, clear codes and verify the repair.

Safety reminders for diesel mechanics

  • Chock wheels and support vehicles with jack stands. Never rely on a jack alone
  • Lock out and tag out vehicles and equipment during repairs
  • Wear eye protection and gloves when working with fluids and pressurized systems
  • Cage tires before inflating split-rim or multi-piece wheels

Compression ignition · DPF · Regeneration (regen) · DEF · SCR · EGR: see the full diesel mechanic glossary.

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