PACCAR MX Engine Overview
The PACCAR MX-11 and MX-13 engines are used in Kenworth and Peterbilt trucks. They are known for reliability but have specific fault patterns that technicians encounter regularly. This guide covers the 10 most common fault codes and how to diagnose them correctly.
1. SPN 5246 — SCR Operator Inducement
The most common PACCAR MX fault. Triggered by DEF quality, SCR efficiency, or NOx sensor issues. Always test DEF quality first before replacing sensors.
2. SPN 4364 FMI 18 — SCR Conversion Efficiency Low
SCR efficiency below threshold. Most commonly caused by bad DEF, clogged dosing injector, or failed NOx sensor. Perform a forced regen and monitor efficiency after DEF quality is confirmed.
3. SPN 157 FMI 3 — Fuel Rail Pressure High
Fuel rail pressure above normal. Check the fuel pressure regulator, injector return lines, and high-pressure pump. On PACCAR MX engines, a stuck-open pressure regulator is a common cause.
4. SPN 3031 — DPF Soot Load High
DPF soot accumulation above threshold. Attempt a forced regen first. If regen fails or soot load returns quickly, check for oil consumption, coolant leaks into exhaust, or short-trip driving patterns.
5. SPN 3216 — DEF Quality Fault
DEF concentration out of spec. Test with a refractometer. Replace DEF if concentration is outside 31.5–33.5%. Check for water contamination in the DEF tank.
6. SPN 100 FMI 1 — Engine Oil Pressure Low
Oil pressure below normal. Check oil level first. If oil level is correct, test oil pressure with a mechanical gauge. Low pressure at idle with correct oil level indicates a worn oil pump or main bearing clearance issue.
7. SPN 1569 — Engine Derate Active
Engine derate is active due to a protection fault. This is a downstream fault — find and fix the root cause fault first. Common triggers include oil pressure, coolant temperature, and aftertreatment faults.
8. SPN 5394 — DEF Dosing System
DEF dosing system fault. Check the dosing injector for crystallization, the DEF pump for correct pressure, and the supply lines for blockages. On PACCAR MX engines, the dosing injector is accessible from the exhaust side.
9. SPN 171 FMI 3 — Turbo Speed Sensor
Turbo speed sensor voltage above normal. Check the sensor connector for corrosion, verify 5V reference and ground, and test sensor resistance. On PACCAR MX engines, the turbo speed sensor is located on the compressor housing.
10. SPN 3226 — NOx Sensor Downstream
Downstream NOx sensor fault. Verify sensor connector integrity, check for exhaust leaks near the sensor, and test sensor heater circuit. Replace the sensor if the heater circuit is open.
PACCAR MX Diagnostic Tips
Always check for multiple active faults before diagnosing. PACCAR MX engines often set cascade faults from a single root cause.
DEF quality is the #1 cause of aftertreatment faults on PACCAR MX engines. Test it first, every time.
PACCAR MX engines have a specific regen inhibit system. If regen is inhibited, find and fix the inhibit cause before attempting a forced regen.
Photo: PACCAR MX-13 engine bay — ECM location and diagnostic port access