Primary fault reference: SPN 5246 FMI 31 — Full Fault Code Reference — This page covers the step-by-step repair workflow. For the complete fault definition, symptoms, causes, and derate risk, see the primary reference above.
Fix SPN 5246 FMI 31 — DEF Inducement / Engine Derate
Step-by-step guide to clearing DEF inducement and restoring full power
Overview
SPN 5246 FMI 31 is a DEF (Diesel Exhaust Fluid) inducement fault that triggers a progressive engine derate. The ECM sets this fault when DEF quality is below threshold, DEF level is critically low, or the SCR system has been non-functional for an extended period. Left unaddressed, the engine will derate to 5 mph.
Safety Warnings
- ⚠Do not attempt to bypass or delete the DEF system — this is illegal under EPA regulations and will result in failed emissions inspections.
- ⚠Wear nitrile gloves when handling DEF — it is a mild skin irritant.
- ⚠Allow exhaust system to cool before inspecting aftertreatment components.
Step-by-Step Repair
Read all active fault codes
Connect OEM diagnostic software and read all active and inactive fault codes. Document every fault — SPN 5246 FMI 31 is often accompanied by DEF quality, level, or dosing faults that must be addressed together.
Check DEF level
Verify DEF tank level. If DEF level is below 10%, fill the tank with fresh, certified DEF (ISO 22241 compliant). Do not use water or non-certified DEF.
Test DEF quality
Use a DEF refractometer to test the urea concentration of the DEF in the tank. Correct concentration is 32.5% ± 1.5%. If concentration is out of range, drain the tank completely and refill with fresh DEF.
Drain and flush tank if DEF is contaminated
If DEF is contaminated (wrong concentration, visible contamination, or diesel fuel in DEF tank), drain the tank completely. Flush with fresh water, then drain again. Refill with fresh, certified DEF.
Clear fault codes and perform SCR system test
After filling with fresh DEF, clear all fault codes with OEM diagnostic software. Perform an SCR system test or DEF dosing test to verify the system is operating correctly.
Perform a drive cycle
Drive the truck at highway speed (55+ mph) for at least 20 minutes to allow the SCR system to reach operating temperature and begin DEF dosing. Monitor NOx sensor readings and DEF dosing rate in live data.
Verify fault does not return
After the drive cycle, check for returning fault codes. If SPN 5246 FMI 31 returns after fresh DEF and a drive cycle, proceed to diagnose the DEF dosing unit, NOx sensors, and SCR catalyst efficiency.
Verification Checklist
- DEF tank filled with certified 32.5% urea solution
- No active fault codes after drive cycle
- DEF dosing rate visible in live data during highway driving
- NOx sensor 1 and 2 readings within normal range
- SCR efficiency above 70% in live data
Related Fault Codes
- SPN 4364 FMI 18 — SCR Efficiency Below Threshold
- SPN 4094 FMI 31 — DEF Quality Below Threshold
- SPN 3364 FMI 2 — DEF Dosing Erratic