How PACCAR MX fault codes work on Peterbilt and Kenworth
PACCAR MX-13 and MX-11 engines store faults as P-codes, such as P1518 or P3796, and that is what a PACCAR tool shows. Behind each P-code sits a standard SPN/FMI pair that the cluster also displays because EPA rules require it. The two formats do not map one-to-one, so give the shop the P-code when you have it. The lamp color still sets the urgency: amber is service soon, red is stop.
P-codes vs SPN/FMI on the PACCAR dash
Diesel Laptops puts it bluntly: PACCAR diagnostics are done with P-codes, not SPN and FMI, and there is no complete published mapping between them. A 2013 and newer Peterbilt 579 or Kenworth T680 shows an SAE-style SPN/FMI on the driver display because the EPA requires it. The dealer's DAVIE software works in P-codes.
- P-code: four characters after the P, for example P0087, P1489, P3796. The description is specific: "DEF level very low" or "EGR valve stuck closed".
- SPN/FMI: the J1939 pair, for example SPN 3251 FMI 0. Useful for cross-brand tools and code lookups.
- Lamps: amber check engine, amber MIL for emissions, red stop engine. Red with a flashing DEF symbol means a speed limit is close.
Photograph both screens if the truck shows them. One will match whatever tool the shop has.
Seeing a light on the dash? Snap it with DERATE and know if you can keep driving
The common MX-13 and MX-11 code families
DEF level and inducement. P1519 is DEF level low, P151D very low, P151E tank empty. When the system acts you see P1518 (SPN 1787 FMI 11), engine torque derate active, then P151A (SPN 74 FMI 14), vehicle speed limitation active. On the original software the steps ended at 5 mph (8 km/h) about 4 hours after a component or fluid-quality fault. PACCAR released new software on July 7, 2026, following revised EPA guidance: the final limit rises to 25 mph (40 km/h) and the delay stretches from 4 hours to 160 hours. Trucks built after 2018 can get it at a Peterbilt or Kenworth dealer. See /guides/def-fault-codes-explained.
DEF quality. P3822 (SPN 3516 FMI 18) is DEF concentration too low: watered-down fluid, wrong fluid, or a bad quality sensor. The MX-13 also has an ammonia (NH3) sensor on the SCR canister, so a doser problem can show up as a quality or efficiency fault first.
DPF soot. P3796 is DPF soot level most severe. P3791 (SPN 5397 FMI 31) is too frequent regeneration, which points at a leaking injector or a sticking EGR valve rather than the filter. A flashing DPF lamp means a parked regen now. Guide: /guides/dpf-regeneration-guide.
EGR. P1489 (SPN 2791 FMI 1) is EGR valve stuck closed. P1480 (SPN 2791 FMI 2) is position unable to reach target. The MX-13 EGR valve is a known wear item. Causes and costs in /guides/egr-valve-and-cooler-problems.
Turbo (VGT). P0046 (SPN 641 FMI 7) is VTG turbocharger actuator position error. P2563 (SPN 2795 FMI 7) is actuator no or too slow movement. Symptoms: no boost up grades, black smoke. Low boost also shows as SPN 102 FMI 18. More in /guides/turbo-failure-signs.
Fuel pressure. P0087 (SPN 94 FMI 1) is fuel pressure too low, most severe. Start with the filters. The MX-13 derates on a restricted filter before it stalls.
Coolant. P0217 (SPN 110) is coolant temperature too high, most severe. Red lamp, stop now. Check the EGR cooler for leaks if you also lose coolant with no puddle.
Keep driving or stop
- Keep driving, book service: single amber lamp with an EGR position, NOx sensor or low DEF level code, no power loss.
- Caution, shop today: P1518 torque derate active, any DEF quality code, flashing DPF lamp. The inducement clock is running.
- Stop: red stop engine lamp, P0217 coolant, oil pressure, or P151A speed limitation already active.
What a driver can check first
- Read the message center. The Peterbilt 579 and Kenworth T680 list active faults with a description. Photograph it, and the lamps.
- DEF: check the gauge and the last fill. If fuel went in the DEF tank or DEF in the fuel tank, stop and call. Do not restart.
- Fuel filters: with a P0087 or power loss, a plugged primary filter is the first suspect.
- Coolant and oil: level and leaks, engine cool.
- Run the photo through DERATE, an iPhone app that reads a picture of the dash warning lights or a scanner screen and gives a free keep-driving, caution or stop verdict. The detailed causes, steps and cost come with a plan.
- For a derate or any aftertreatment fault, go to a dealer with DAVIE or a shop with a dealer-level tool. Only those clear the inducement after the repair. A handheld that only clears codes buys hours and hides the evidence the shop needs.
Costs vary by shop and region. Fuel filters: about 100 to 250 dollars. MX-13 EGR valve: typically 1,200 to 2,500 dollars installed. VGT actuator: about 1,500 to 3,000 dollars. DPF clean: typically 300 to 600 dollars.
FAQ
Why does my Kenworth show an SPN but the dealer talks about a P-code?
The cluster shows the SAE SPN/FMI because the EPA requires it. PACCAR's own tool, DAVIE, works in P-codes. Give the dealer the exact text and both numbers if you have them.
Can I keep driving with P1518 torque derate active?
You can move the truck, but the inducement is counting. Fix the root DEF or SCR fault within hours, not days, or P151A speed limitation follows.
Is the 5 mph DEF derate still a thing on PACCAR MX engines?
On trucks with the original software, yes. PACCAR's July 2026 software update raises the final limit to 25 mph on updated MX-11 and MX-13 engines. Ask the dealer whether your truck has it.