PACCAR MX-13 and MX-11 engines power most modern Kenworth T680/T880 and Peterbilt 579/567 chassis. The 15 codes below are the ones that show up consistently in independent shop work. SPN/FMI format. For full reflash and dealer-level access on Kenworth or Peterbilt, you need PACCAR Davie4 or Davie5 — see Davie packages.
Fuel system codes
| SPN/FMI |
Description |
Common cause |
Next step |
| 164/0 |
Fuel rail pressure too high |
Stuck pressure regulator, failed rail pressure sensor |
Live data — commanded vs actual rail pressure under load. Replace pressure regulator if actual exceeds commanded persistently. |
| 164/1 |
Fuel rail pressure too low |
Plugged fuel filter, weak lift pump, dropping injector, air in system |
Filter restriction check, lift pump pressure check, then cylinder cutout via Davie if both OK. |
| 651-656/5 or 7 |
Cylinder N injector circuit / response fault |
Failed injector, harness chafe, ECM driver fault |
Davie injector solenoid test. Swap suspect injector with known-good cylinder; if fault follows the injector, replace. |
Air handling codes
| SPN/FMI |
Description |
Common cause |
Next step |
| 411/4 or 5 |
EGR valve position circuit fault |
Carbon buildup on EGR valve, failed position sensor |
EGR valve sweep test in Davie. Decarbon or replace if mechanical bind. |
| 2789/14 |
VGT actuator commanded position not reached |
Carbon buildup on VGT vanes, failed VGT actuator |
VGT sweep test. Decarbon or replace turbo if vanes are stuck; replace actuator only if electrical. |
| 27/4 |
Boost pressure sensor circuit fault |
Failed boost sensor, oil contamination of sensor port (worn turbo seals) |
Replace sensor; check intake for excessive oil indicating turbo seal wear. |
Aftertreatment codes
| SPN/FMI |
Description |
Common cause |
Next step |
| 3251/0 |
DPF differential pressure too high |
Heavy soot loading, failed dP sensor, plugged dP lines |
Force regen via Davie. If dP doesn’t drop after regen, DPF needs cleaning or replacement. |
| 3361/5 or 7 |
DEF dosing valve / injector fault |
Crystallized DEF in line, failed dosing valve |
Dosing test in Davie. Inspect dosing line for white DEF crystal buildup. |
| 3936/0 |
SCR conversion efficiency low |
Failed NOx sensor, expired DEF, EGR over-flow, intake leak |
Compare upstream/downstream NOx sensors. Verify DEF concentration. NOx sensors are a wear item on MX-13 around 250-400K miles. |
| 4334/0 or 1 |
DEF pump pressure abnormal |
Frozen DEF lines, failed DEF pump, low DEF |
Check DEF heater operation in winter. Cycle ignition while monitoring pump pressure. |
| 5246/0 or 16 |
Aftertreatment derate active / inducement |
Persistent SCR or DEF fault |
Find the root SCR/DEF cause; this code is downstream. Inducement reset may require under-load drive cycle. |
Sensors and electrical
| SPN/FMI |
Description |
Common cause |
Next step |
| 110/3 or 4 |
Coolant temperature sensor circuit fault |
Failed ECT, harness chafe, corroded connector |
Backprobe sensor; replace if out-of-spec resistance, repair harness if voltage doesn’t match. |
| 100/1 or 4 |
Engine oil pressure low / sensor fault |
Genuinely low oil pressure or failed sensor |
Confirm with mechanical gauge before assuming sensor failure. |
| 168/0 or 1 |
Battery voltage out of range |
Failing batteries, alternator over/under-charging, ground corrosion |
Check batteries under load, alternator output, all chassis grounds. PACCAR is sensitive to weak ground straps. |
PACCAR-specific notes
- Davie4 vs Davie5: PACCAR is rolling Davie5 in as the new diagnostic platform. Late-model trucks may require Davie5; older trucks (pre-2023) work with Davie4. Davie5 software requires a separate license. Davie4 still covers the bulk of the existing fleet through 2023-2024 models.
- Kenworth vs Peterbilt: Same engine, slightly different chassis modules. Davie covers both brands from a single license.
- Adapter compatibility: NEXIQ USB-Link 3 works with Davie. PACCAR’s own dealer adapters work too, but USB-Link 3 is the most common shop choice.
Tools we recommend
- PACCAR Davie4 or Davie5 — full diagnostic, reflash, programming. Browse Davie packages
- NEXIQ USB-Link 3 — RP1210 datalink adapter
- JPRO Professional — strong PACCAR alternative for diagnose / regen / bidirectional without reflash. JPRO packages
- DEF refractometer — quickly verifies DEF concentration on suspected SCR cases
Frequently asked questions
Stuck on an MX-13 code? Free help.
Tell us the SPN/FMI, what’s been replaced, and what live data you’re seeing. Free phone diagnostic guidance — call (800) 399-9495 or browse PACCAR Davie packages.
