International DEF System Fault — Causes & Fixes

What Does This Error Mean?

An International DEF system fault means the SCR has detected low fluid, contamination, or a dosing fault. The truck derates within hours if ignored. Resolve the issue immediately.

Affected Models

International LT
International RH
International HV
International HX
International MV
International CV

Common Causes

DEF tank low or empty
Contaminated or expired DEF (over 12 months old)
DEF crystallisation in the dosing line or injector
Failed DEF quality sensor
Failed DEF pump, dosing valve, or NOx sensor

How to Fix It

Check DEF level and quality.

Verify the DEF tank is at least one-quarter full with fresh DEF. DEF older than 12 months degrades and can trigger quality faults. Use only ISO 22241-compliant DEF.
Inspect for crystallisation.

Look around the DEF dosing valve, injector, and connecting lines for white crystal deposits. Crystallisation blocks dosing. Cleaning is best done by a qualified diesel technician.
Connect a J1939 scan tool.

A scan tool reads the specific DEF system fault code. The code identifies which component (pump, sensor, injector, or NOx sensor) is at fault.
Address before derate.

International DEF faults escalate over hours: warning → power reduction → 5 mph derate. Resolve the issue before reaching the derate stage. After repair, a regen or driving cycle may be needed for full clear.

When to Call a Professional

Navistar's ServiceMaxx and Diamond Logic Builder tools provide detailed DEF system diagnostics. An International dealer or Navistar-authorised shop can identify the failed component and perform repair.