SPN 4732 FMI 2 — DEF quality sensor — erratic readings
Applies to: Navistar · Paccar · Sae · Volvo
DEF quality sensor — erratic readings.
Your SPN/FMI rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Contaminated DEF (water
- fuel
- washer fluid)
- Diluted DEF
- Failed quality sensor
- Contaminated/diluted DEF
- Sensor
First moves (roadside)
- Refractometer test — should read 32.5% urea
- Drain and refill if contaminated
- Refractometer test
- Test DEF with refractometer (32.5% urea)
Common misdiagnoses
- Replacing the sensor when DEF is actually bad
- Replacing sensor without testing
- Replacing sensor without testing DEF
FAQ
DEF quality sensor — erratic readings.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Contaminated DEF (water. Other possibilities include fuel; washer fluid); Diluted DEF; Failed quality sensor; Contaminated/diluted DEF; Sensor.
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.