Diesel Diagnostics · PACCAR MX-13
PACCAR MX-13 Fault Codes: How to Read and Diagnose Them
MX-13 faults are reported as SPN and FMI pairs like every other J1939 engine, plus a PACCAR-specific fault number in the dash and service tool. The number tells you where to look; it does not tell you what failed. This is how to work them.
Written and reviewed by Christopher Scott Sneed, 30+ year diesel technician, founder of Blue Collar Empire. Updated 2026-08-28.
Get the fault, the SPN/FMI, and the count
Record the PACCAR fault number together with its SPN and FMI, whether it is active or inactive, and the occurrence count. High-count inactive faults that keep returning matter more than a one-time active code set during a jump start.
Systems that generate most MX-13 faults
- Aftertreatment — DPF differential pressure, exhaust temperature sensors, DEF dosing and quality, SCR conversion efficiency.
- EGR — valve position, cooler efficiency, differential pressure.
- Air management — charge-air leaks, turbo actuator position faults.
- Fuel — rail pressure control, injector circuit faults.
- Electrical — sensor supply and J1939 communication faults, often traced to grounds.
Aftertreatment codes: verify sensors before parts
Exhaust temperature sensors on this engine are a frequent source of nuisance codes. At a cold soak, every exhaust temperature sensor should read within a few degrees of ambient and of each other. Any sensor that disagrees at a cold soak is bad, and it will distort regen control and SCR dosing until it is replaced.
Communication and power faults
MX-13 trucks are sensitive to charging-system and ground problems. Before chasing a module fault, verify charging voltage under load, check the battery box terminals and the frame ground, and confirm 60 ohms across the J1939 bus. Several unrelated module faults at once is a power or bus problem until proven otherwise.
Repair considerations
Clear codes only after the repair, then run the truck through the operating range that set them and re-read. Aftertreatment repairs often need a completed regen or a dosing validation cycle before the ECM will drop a derate. Document the tests — the next tech to see this truck should not have to start over.
Tools that do this work for you
Everything above is included in the Blue Collar Empire membership — $14.99 a month.
Frequently asked questions
- Where do I find PACCAR MX-13 fault codes?
- In the dash display fault menu and in the service tool. Record the PACCAR fault number along with the J1939 SPN and FMI — you need both to look the fault up accurately.
- Do MX-13 fault codes clear themselves?
- Active faults go inactive when the condition stops, but the history and occurrence count stay stored. A rising count on an inactive fault means the problem is still there.