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DEF System Fault — Causes, Fault Codes & How to Fix

DEF (Diesel Exhaust Fluid) system faults are the leading cause of truck derate and inducement. This guide covers all major DEF/SCR fault codes with diagnostic procedures to clear the fault before it escalates to a 5 mph speed limit.

DEF system fault diagnostic illustration

Inducement Warning

DEF system faults follow a graduated response: warning → derate → 5 mph limit → idle-only. Address DEF faults immediately to prevent inducement. Once at Severity 3 (idle-only), dealer-level reset may be required.

First Steps for Any DEF Fault

1
Check DEF level
Refill DEF tank if low. Use only ISO 22241-compliant DEF.
2
Test DEF quality
Use a refractometer to verify urea concentration is 31.8–33.2%. Diluted or contaminated DEF is the most common cause.
3
Read all active fault codes
Use OEM software to identify the specific DEF/SCR fault code. Multiple codes may be active.
4
Check DEF temperature
DEF freezes below 12°F. If operating in cold weather, verify DEF heater is functioning.
5
Address the specific fault code
Use the fault code links below to access full diagnostic procedures.