FC 1682 — Aftertreatment DEF Dosing Unit Input Lines – Condition Exists (SPN 3362 FMI 31) — Cummins ISX15

Diagnostic reference for Cummins ISX15 (2010–2017) FC 1682.

Updated October 15, 2026

What it means

FC 1682 is SPN 3362 FMI 31. The ECM detected a problem in the DEF dosing unit supply (input) lines.¹

Most likely causes

1. Restricted, frozen or kinked DEF supply line to the dosing unit — Physical blockage or ice in the input line prevents proper DEF delivery to the doser.¹
2. Clogged DEF filter or strainer upstream of the dosing unit — Debris or crystallization restricts flow into the dosing unit.¹
3. Low DEF level affecting supply to the dosing unit — Insufficient fluid in the tank can cause the input line condition to be flagged.¹
4. DEF pressure or dosing unit related issue — Erratic or implausible DEF pressure can accompany input line faults.²

How to diagnose

1. Verify DEF tank level and recent refill history; low level can trigger supply line condition.
2. Inspect DEF supply line from tank to dosing unit for kinks, crushing, or damage; check for freezing conditions.
3. Check DEF filter/strainer for clogging or crystallization; replace if restricted.
4. Review active and inactive codes for DEF pressure erratic/intermittent (FC 3596) or DEF quality below normal (FC 3867) that may contribute.
5. If SCR efficiency codes (e.g., FC 3582) or inducement codes (FC 6254, FC 3712) are present, address the underlying DEF dosing issue first; inducement is an effect, not a root cause.

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

• DEF filter/strainer
• DEF supply line
• DEF tank level sender
• Diagnostic scan tool for Cummins FC/SPN/FMI

Tools you’ll need

• digital multimeter — Checks continuity and voltage on DEF dosing unit supply line sensors and wiring to confirm electrical integrity.
• OBD-II scan tool — Reads SPN 3362 FMI 31 and related DEF pressure/quality codes to confirm active faults and monitor live data.
• inspection mirror — Allows visual inspection of DEF supply line routing and connections in tight engine bay areas for kinks or damage.
• DEF refractometer — Verifies DEF concentration to rule out crystallization or poor quality contributing to supply restriction.

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 inducement derate is active (FC 3712/3714) or the fault persists after line/filter inspection, have a Cummins-certified technician diagnose with Insite to confirm dosing unit health and avoid unnecessary parts replacement.

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