SPN 4094 FMI 31 — DEF Quality — Out of Spec

What SPN 4094 FMI 31 means

The system has determined the DEF doesn't meet the required quality spec (the fluid should be ~32.5% urea). Poor DEF quality cripples NOx reduction and triggers an inducement/derate countdown if not corrected.

How serious is it?

Serious. Power derate likely — plan to stop soon.

Most likely causes

1. Contaminated, diluted, expired, or counterfeit DEF in the tank
2. Water or fuel accidentally added to the DEF tank
3. DEF quality sensor fault reporting bad fluid
4. Old DEF that degraded from heat or long storage

First moves (roadside)

1. Test or refractometer-check the DEF; if in doubt, drain and refill with fresh in-spec DEF.
2. Make sure nothing but DEF went in the tank — water or diesel will fail quality immediately.
3. After refilling with good fluid, the system needs to re-evaluate quality over a drive cycle.
4. If fresh, verified DEF still fails, suspect the quality sensor.

Common misdiagnoses

• Assuming the sensor is bad when the DEF was actually contaminated or diluted.
• Topping off bad DEF with good DEF instead of draining — it still reads out of spec.
• Ignoring it until the inducement derate caps the truck's speed.

FAQ

Related codes

Codes that commonly appear alongside SPN 4094 FMI 31, or share the same subsystem.

• SCR Operator Inducement — Severe Derate Active (SPN 5246 FMI 0)
• Aftertreatment 1 Diesel Particulate Filter Differential Pressure - Data Valid But Above Normal Operating Range, Most Severe Level (DPF severely restricted / plugged) (SPN 3251 FMI 0)
• Aftertreatment 1 Diesel Exhaust Fluid Quality - Data Valid But Below Normal Operating Range, Most Severe Level (DEF quality/concentration incorrect) (SPN 4364 FMI 1)
• DPF Differential Pressure — Data Erratic (SPN 3251 FMI 2)
• DEF Dosing Unit — Not Responding Properly (SPN 3361 FMI 7)
• SCR Conversion Efficiency — Below Threshold (SPN 4364 FMI 18)
• EGR Temperature — Too High (SPN 412 FMI 0)
• DEF Dosing Valve — Not Responding (SPN 5394 FMI 7)

Same symptom: DEF / SCR fault

Other codes in the DEF and SCR chain — the ones that end in an inducement countdown.

• Reductant System Performance Bank 1 (P204F) · Aftertreatment & Emissions
• Reductant Quality Deviation (P20EA) · Aftertreatment & Emissions
• DEF tank level low (SPN 1761 FMI 18) · Aftertreatment & Emissions
• DEF tank temp sensor — voltage high (SPN 3031 FMI 3) · Aftertreatment & Emissions
• Upstream NOx sensor — internal failure reported (SPN 3216 FMI 12) · Aftertreatment & Emissions
• SCR inlet NOx sensor — erratic (SPN 3216 FMI 2) · Aftertreatment & Emissions

All Aftertreatment & Emissions codes

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