SPN 1761 FMI 1 — DEF Tank Level — Low

What SPN 1761 FMI 1 means

Diesel exhaust fluid (DEF) tank level is low. Early on this is just a warning, but if you run the tank dry the truck will stage warnings and ultimately a speed-limiting inducement derate. Easy to fix if you catch it in time.

How serious is it?

Caution. Usually OK to continue — monitor closely.

Most likely causes

1. DEF tank genuinely low — needs a refill
2. Faulty DEF level/quality sensor reading low
3. DEF header/sender wiring or connector fault
4. Frozen DEF in cold weather reading as low/empty

First moves (roadside)

1. Add DEF — refilling is the first and usually the only step needed for a true low-level warning.
2. Use fresh, in-spec DEF (don't dilute with water or use contaminated fluid).
3. If the tank is actually full but it still reads low, suspect the level sensor/wiring.
4. In freezing temps, allow the DEF heater to thaw the system before condemning the sensor.

Common misdiagnoses

• Replacing the level sensor when the tank was simply low.
• Topping off with water or off-spec fluid, which then triggers a DEF-quality fault.
• Ignoring the warning until it escalates into a speed-limiting inducement derate.

FAQ

Related codes

Codes that commonly appear alongside SPN 1761 FMI 1, or share the same subsystem.

• DEF tank level low (SPN 1761 FMI 18)
• 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)
• DEF Quality — Out of Spec (SPN 4094 FMI 31)

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 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
• Upstream NOx sensor — abnormal update rate (SPN 3216 FMI 9) · Aftertreatment & Emissions

All Aftertreatment & Emissions codes

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