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Fault Code · Cummins

SPN 3216 FMI 4 — Cummins

Aftertreatment 1 intake NOx sensor — voltage below normal or shorted to low source. Symptoms, ranked causes, step-by-step diagnostics and the tools that verify the repair.

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What SPN 3216 FMI 4 means

SPN 3216 FMI 4: Aftertreatment 1 intake NOx sensor — voltage below normal or shorted to low source.

Applies to: ISX15, X15, X12, B6.7, L9 (CM2250/CM2350/CM2450), 2010–2026.

Severity: High — contributes to SCR inducement countdown.

Symptoms you'll see

  • MIL illuminated
  • SCR/DEF warning on dash
  • Possible derate countdown if paired with SPN 5246
  • NOx readings frozen or implausible on scan data

Likely causes (ranked by probability)

  • Failed NOx sensor element (~60% of cases) — exposure to thermal cycling and moisture
  • Chafed or corroded sensor harness, especially at the frame-rail routing (~25%)
  • Water intrusion in the sensor connector (~10%)
  • ECU-side fault — rare, verify everything else first (~5%)
Diagnose, don't guess. Aftertreatment parts are expensive and the ranked list above is where they get wasted — follow the steps below before replacing anything.

Diagnostic steps

  • Read freeze-frame: confirm the code set under load, not at key-on (intermittent harness hint).
  • Inspect the sensor connector for green corrosion and the harness for rub points at the frame rail.
  • Wiggle-test with live data in INSITE — voltage dropping out confirms harness over sensor.
  • Substitute a known-good sensor if available; otherwise ohm the harness per the Cummins spec.
  • Replace sensor or repair harness, clear codes, run SCR system test to verify.

Tools that read, clear and verify this code

Reads/clears with any J1939 scanner; INSITE's NOx sensor test and guided tree cut diagnosis time in half. For the full per-platform picture see the Cummins ISX fault codes page and our Aftertreatment fault codes.

Frequently Asked Questions

Can I drive with SPN 3216 FMI 4 active?

Severity on this code: High — contributes to SCR inducement countdown. If a derate countdown is armed, use the remaining runtime to reach a shop — not to finish the week. Verify the root cause with the diagnostic steps above rather than clearing and hoping.

Will clearing SPN 3216 FMI 4 without repairs work?

The code will return — usually within one drive cycle — and on aftertreatment-related faults the inducement countdown often shortens each time. Repair first, verify with the guided test, then clear.

Which scanner reads SPN 3216 FMI 4?

Reads/clears with any J1939 scanner; INSITE's NOx sensor test and guided tree cut diagnosis time in half.

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