FC 4677 — Aftertreatment DEF Tank Level – Abnormal Update Rate (SPN 1761 FMI 9) — Cummins ISX15

Cummins ISX15 (2010–2017) diagnostic reference for FC 4677 (SPN 1761 FMI 9).

Updated October 15, 2026

What it means

The DEF tank level signal is not updating at the expected rate. On Cummins ISX15 (2010–2017) this maps to SPN 1761 FMI 9 and is associated with the DEF tank level/temperature/quality sender or its wiring/datalink.¹

Most likely causes

1. DEF tank level/temperature/quality sender (combined sensor) intermittent or failing — The sender assembly provides level, temperature and quality data to the ECM. An intermittent or failing sender can produce an abnormal update rate for the level signal.¹
2. Wiring or connector fault at the DEF tank sender or chassis harness — Corrosion, chafing, loose pins or moisture in the DEF tank sender connector or the harness to the ECM can cause sporadic communication and missed updates.¹
3. Datalink / CAN communication issue affecting the DEF tank module signal — A network fault or poor termination can delay or drop messages from the DEF tank sender, resulting in an abnormal update rate.¹
4. DEF tank heater or temperature-related freeze condition affecting sensor readings — In cold weather, frozen DEF or a heater circuit issue can cause erratic sensor behavior and update anomalies.²³
5. Contaminated or diluted DEF causing sensor/quality logic interference — Poor DEF quality can trigger related sensor faults and influence level/quality reporting on the ISX15.⁴

How to diagnose

1. Verify active vs inactive status and freeze-frame conditions for FC 4677 (SPN 1761 FMI 9).
2. Inspect DEF tank sender connector and harness for corrosion, moisture, bent pins, and chafing; repair as needed.
3. Check DEF tank sender supply voltage and ground at the connector per service data; look for intermittent opens.
4. Use diagnostic tool to monitor live DEF tank level, temperature and quality PIDs while gently moving the harness to reveal intermittents.
5. Inspect DEF tank heater operation and coolant flow in cold weather; confirm heater circuit integrity if freeze is suspected.
6. Check for companion codes such as DEF temperature abnormal update rate, DEF quality faults, or SCR inducement codes that may indicate a broader DEF system issue.
7. Clear codes after repairs and road test to confirm the level signal updates at the expected rate and the fault does not return.

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 with Cummins INSITE or equivalent J1939 reader
• Digital multimeter
• Terminal release tools
• Dielectric grease
• Replacement DEF tank level/temperature/quality sender assembly
• Wiring repair kit (sealed splices, heat-shrink)
• Service information for CM2250/CM2350 pinouts

Tools you’ll need

• diagnostic scan tool — Reads SPN 1761 FMI 9 and live DEF tank level/temperature/quality PIDs to confirm the slow-update condition and monitor signal behavior during harness wiggle tests.
• digital multimeter — Checks DEF tank sender supply voltage, ground integrity, and continuity of the sensor and CAN wiring to find intermittent opens or corrosion causing the slow update.
• terminal release tool set — Allows safe inspection and reseating of the DEF tank sender connector pins without damage to diagnose corrosion or bent-pin faults.
• inspection mirror and flashlight — Provides visibility of the DEF tank sender connector, harness chafing, and heater components in tight under-vehicle areas for corrosion and moisture checks.

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 the fault persists after connector and harness checks, or if SCR operator inducement/derate codes appear, have a qualified diesel technician diagnose the DEF system with OEM software to avoid unnecessary parts replacement and potential derate escalation.

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