SPN 4094 FMI 31 — DEF Quality — Out of Spec
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.
Look up listed providers near you and give them this SPN/FMI when you call.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Contaminated, diluted, expired, or counterfeit DEF in the tank
- Water or fuel accidentally added to the DEF tank
- DEF quality sensor fault reporting bad fluid
- Old DEF that degraded from heat or long storage
First moves (roadside)
- Test or refractometer-check the DEF; if in doubt, drain and refill with fresh in-spec DEF.
- Make sure nothing but DEF went in the tank — water or diesel will fail quality immediately.
- After refilling with good fluid, the system needs to re-evaluate quality over a drive cycle.
- 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
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.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Contaminated, diluted, expired, or counterfeit DEF in the tank. Other possibilities include Water or fuel accidentally added to the DEF tank; DEF quality sensor fault reporting bad fluid; Old DEF that degraded from heat or long storage.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.