What’s Up With All the Different Fault Code Numbers?

by Jake Schell

On-board diagnostic codes have been part of the automotive world for a long time. During the 1990s, On-board diagnostics (OBD I and then OBD II) started to be an everyday aspect for automotive repair shops working on cars and light-duty trucks. Virtually all technicians today recognize the OBD II codes when read through a scan tool as well as when navigating repair information to diagnose and repair the truck fault code. On the other hand, some light-duty shops will bring in medium trucks and note some very different and differing code naming structures when the codes are read on a dash display or with a scan tool made for reading J1587 or J1939 codes.

The heavy truck codes, SAE J1587 or J1939, along with the OE codes, are particularly confusing when looking up service information for the uninitiated. If you are new to these code structures, this article will hopefully make it easier for you to look up these codes.

The OE codes that may display on the dash are typically 3-4 digits; however, the J1587/J1939 codes, which display on aftermarket heavy truck scan tools, consist of three sections. Here are examples from TruckSeries’ RepairConnect:

For J1587, the first string is the Message Identifier (MID), then either a Parameter Identifier (PID) or Subsystem Identifier (SID) and, lastly, a Failure Mode Identifier (FMI).

For J1939, the first string is the Source Address (SA), then the Suspect Parameter Number (SPN) and the Failure Mode Identifier (FMI).

When using TractorTrailer or MediumTruck, the codes are laid out a little differently. Navigating to Engine Performance, the user will see a list of Fault Code titles.

The titles are listed by the OE code numbers. Opening up the first on the list, the Tech is confronted with a long list of OE codes that may not be understood because the Tech’s scan tool displays the J1587 or J1939 codes.

To see the J1587 or J1939 codes, select the link under the FAULT CODE INDEX.

A table will open up that lists all the protocols.

Dealing with truck codes may at first seem rather forbidding, but with a little insight into how to navigate them, you will quickly discover that they really are not all that difficult to understand and navigate.

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About the Author

Jake Schell

Jake Schell is an editorial consultant with Mitchell 1. Previously, he served as Product Manager for the Commercial Vehicle Group from 2002 to 2023. Prior to joining Mitchell 1, Jake spent 20 years as a technician. He holds a Chevrolet Master certification in the transmission category as well as ASE certifications in both cars and trucks.