FC 3582 — Aftertreatment SCR Catalyst Conversion Efficiency – Below Normal (SPN 4364 FMI 18) — Cummins ISX15

Diagnostic reference for Cummins ISX15 (2010–2017) FC 3582.

Updated October 15, 2026

What it means

SCR NOx conversion efficiency is below the expected level. On Cummins ISX15 (2010–2017) this maps to FC 3582 = SPN 4364 FMI 18 and involves the SCR, NOx sensors, and DEF dosing.¹

Most likely causes

1. Poor DEF quality or contaminated reagent — Diluted or contaminated DEF reduces conversion efficiency and can trigger SCR efficiency faults. Check DEF quality and recent fill history.¹²
2. Clogged or crystallized DEF doser / dosing unit restriction — A restricted doser or crystallized injector reduces DEF delivery to the SCR, lowering conversion efficiency.¹
3. NOx sensor fault or drift (inlet/outlet) — Erroneous NOx sensor readings cause the ECM to calculate low conversion efficiency. Compare sensor behavior and rationality.¹
4. Exhaust leak upstream or at SCR inlet/outlet — False air entering the exhaust skews NOx measurements and conversion calculations.¹
5. DEF dosing system pressure or supply issue — Erratic or low DEF pressure from dosing unit, lines, or filters reduces actual dosing. Review DEF pressure behavior.³
6. Low DEF tank level or sender error — Insufficient reagent volume can limit dosing. Verify actual level vs. reported level.⁴
7. DEF quality sensor reporting below normal — A DEF quality sensor fault can contribute to reduced dosing authority and efficiency codes.⁵
8. Operator inducement/derate active due to underlying SCR fault — FC 3712/3714 are effects of an unresolved SCR/DEF fault; the root cause must be found first.⁶⁷

How to diagnose

1. Confirm FC 3582 is active and note accompanying SPNs/FMIs; check for inducement codes FC 3712/6254 and derate FC 3714.
2. Review recent DEF fill history and test DEF quality; look for dilution/contamination.
3. Inspect DEF tank level reading vs actual level; check level sender and heater operation.
4. Check DEF dosing system status: pressure plausibility, dosing unit operation, and signs of crystallization or restriction in lines/filters.
5. Evaluate NOx sensor data: compare inlet vs outlet readings at steady cruise and during regen; check sensor rationality and response.
6. Inspect exhaust system for leaks at DOC/DPF/SCR connections and gaskets that could admit air.
7. Review ECM snapshots for engine load, speed, exhaust temps, and DEF dosing quantity during the fault event.

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 scan tool capable of Cummins FC/SPN-FMI and live data
• DEF refractometer or test strips
• Exhaust backpressure/temperature data via scan tool

Tools you’ll need

• Cummins Insite diagnostic software — Reads FC 3582/SPN 4364 FMI 18 and related inducement codes, plus live NOx sensor and DEF dosing data for this ISX15.
• DEF refractometer — Verifies DEF concentration to rule out poor quality or contamination causing low SCR conversion.
• Digital multimeter — Checks NOx sensor heater and signal circuits and DEF level sender resistance for faults.
• Borescope — Inspects DEF doser nozzle and dosing lines for crystallization/restriction without disassembly.

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 operator inducement or derate is active, or if SCR/DEF repairs are required, have a qualified diesel technician diagnose and service the aftertreatment system. Do not attempt high-pressure fuel or safety-critical repairs yourself.

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