FC 3867 — Aftertreatment DEF Quality – Below Normal (SPN 3364 FMI 18) — Cummins ISX15

Diagnostic reference for Cummins ISX15 (2010–2017) FC 3867.

Updated October 15, 2026

What it means

The DEF quality sensor reports fluid quality below the acceptable range. On Cummins ISX15 (2010–2017) with DOC + DPF + SCR and DEF, this is SPN 3364 FMI 18. Typical on this platform: diluted or contaminated DEF, or a DEF quality/level sensor problem.¹

Most likely causes

1. Diluted or contaminated DEF in the tank — Water, fuel, or other contamination lowers urea concentration and triggers the quality sensor. Check DEF source and recent fill history.¹
2. DEF quality/level sensor or tank module fault — Erroneous quality reading from a faulty sensor, temperature/level module, or wiring/datalink issue.¹
3. DEF tank level/temperature/quality sender or wiring/datalink fault — Abnormal update rate or signal issues from the tank sender can affect quality reporting.²

How to diagnose

1. Verify active vs inactive status and freeze-frame conditions (ambient temp, recent DEF fill, mileage since fill).
2. Inspect DEF tank for contamination, stratification, or incorrect fluid; confirm correct ISO 22241 DEF and no recent mixing or water ingress.
3. Check DEF tank level reading and recent level history; low or erratic level can accompany quality faults.
4. Review companion SCR/DEF codes (e.g., dosing unit input line condition, SCR conversion efficiency) to separate root cause from effects.
5. Inspect DEF tank module connectors and wiring for corrosion, moisture, or chafing; check datalink integrity for abnormal update rate symptoms.
6. Use service tool to view live DEF quality sensor parameters (quality %, temperature) and compare to expected range under stable conditions.
7. If quality reads low with known-good fresh DEF, suspect sensor/module performance; if quality normalizes after a controlled DEF replacement and tank rinse, suspect fluid contamination.

Specs and test values

Common mistake to avoid

Replacing sensors, the doser or the SCR catalyst before testing the DEF. Old, heat-stored or diluted DEF loses strength and sets the same efficiency codes. Check it with a refractometer first. Source: Ram service bulletin 25-005-14 (Diesel Exhaust Fluid).

Parts and tools you may need

• Diagnostic service tool capable of Cummins FC/SPN/FMI read and live data
• Multimeter
• Inspection light and mirror
• Clean containers for DEF sampling

Tools you’ll need

• Cummins Insite diagnostic software — Reads SPN 3364 FMI 18 live data and freeze-frame to confirm active DEF quality fault and view quality %, temperature, and level parameters.
• digital multimeter — Checks DEF tank module connector voltage, ground, and datalink signal integrity for corrosion or wiring faults affecting quality reporting.
• refractometer — Measures DEF urea concentration to verify ISO 22241 compliance and detect dilution or contamination causing low quality readings.
• DEF test strips — Provides a quick field check of urea concentration and contamination in the tank before replacing fluid or the sensor module.

Rough cost and difficulty

Typical US independent-shop figures; labor runs about $140–$200 per hour (dealers charge more). Confirm the diagnosis before buying parts.

When to see a pro

If SCR operator inducement codes appear (e.g., severe derate/speed limit) or the fault persists after fluid verification, have a certified diesel technician diagnose the DEF quality sensor, tank module, and SCR system. Do not attempt high-pressure fuel or aftertreatment service without proper training and equipment.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources