What SPN 3226 FMI 4 means
SPN 3226 FMI 4: Aftertreatment 1 outlet NOx sensor — voltage below normal or shorted to low source.
Applies to: ISX15, X15, X12, B6.7, L9 with SCR, 2010–2026.
Severity: High — SCR efficiency monitoring fails; inducement risk.
Symptoms you'll see
- MIL on
- SCR efficiency codes may follow
- NOx conversion readings drop out on live data
Likely causes (ranked by probability)
- Failed outlet NOx sensor (~65%) — the downstream sensor lives in the harshest exhaust environment
- Harness damage at the SCR outlet routing (~20%)
- Connector corrosion/water intrusion (~10%)
- SCR catalyst damage causing implausible readings that mimic sensor failure (~5%)
Diagnostic steps
- Compare inlet vs outlet NOx on live data — both implausible suggests catalyst issue, one implausible suggests sensor.
- Inspect harness and connector at the SCR outlet.
- Swap-test with a known-good sensor (inlet/outlet sensors are often the same part number).
- Replace and verify with SCR system test.
Tools that read, clear and verify this code
Any J1939 scanner reads it; INSITE's inlet/outlet comparison view is the fastest path to certainty. 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 3226 FMI 4 active?
Severity on this code: High — SCR efficiency monitoring fails; inducement 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 3226 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 3226 FMI 4?
Any J1939 scanner reads it; INSITE's inlet/outlet comparison view is the fastest path to certainty.