What SPN 3216 FMI 4 means
SPN 3216 FMI 4: Aftertreatment intake NOx sensor — voltage below normal.
Applies to: DD13, DD15, DD16 (EPA10 and later incl. Gen 5), 2010–2026.
Severity: High — SCR monitoring compromised, derate risk.
Symptoms you'll see
- MIL
- One-box aftertreatment warning
- NOx data implausible in DDDL
Likely causes (ranked by probability)
- Failed NOx sensor (~60%)
- Harness chafe along the one-box routing (~25%)
- Connector water intrusion (~10%)
- Module fault (~5%)
Diagnostic steps
- Read in DDDL and note whether inlet and outlet codes coexist (both = wiring/power issue more likely).
- Inspect harness at the one-box inlet routing — a known chafe point.
- Run DDDL's NOx sensor plausibility test.
- Replace sensor, clear, verify with the aftertreatment system test.
Tools that read, clear and verify this code
DDDL guided test; readable/clearable on all major CV scanners. For the full per-platform picture see the Detroit DD15 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 — SCR monitoring compromised, derate risk. 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?
DDDL guided test; readable/clearable on all major CV scanners.