SPN 3226 FMI 4: Aftertreatment outlet NOx sensor voltage low – poor NOx data. Symptoms (3), causes (3), and fixes (3). What to check first to prevent derate and downtime.
| Code | SPN 3226 FMI 4 |
|---|---|
| Severity | Warning |
| Applies to | Varies by OEM configuration (confirm your exact calibration) |
| Can I drive? | Usually yes for a short distance, but fix it soon. Warning faults like SPN 3226 FMI 4 often escalate into torque derate/inducement if the condition repeats over multiple drive cycles. The scan-tool checks below help determine how urgent it is. |
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When SPN 3226 FMI 4 sets, the ECM is telling you it detected DEF dosing performance that doesn’t match what it expects for emissions control.
That usually means the system commanded dosing, but the outcome (NOx conversion / feedback) didn’t line up. The root cause can be mechanical (restriction/crystallization), electrical (connector/harness), or fluid-related (quality/contamination).
The exact triggers vary by OEM calibration, but the diagnostic logic is consistent.
The key is to confirm whether the fault is ACTIVE and repeatable right now, or stored from a past event.
The aftertreatment system reduces NOx by injecting DEF upstream of the SCR catalyst. The DEF decomposes into ammonia, and the SCR catalyst uses it to convert NOx into harmless nitrogen and water.
If the ECM commands DEF dosing but doesn’t see the expected result (from NOx sensors and model-based checks), it assumes dosing performance is abnormal.
Common disruptors include crystallized DEF around the doser tip, restricted lines, connector corrosion, or poor fluid quality that prevents consistent dosing.
This appears when the SCR outlet NOx sensor reports voltage lower than expected. It often shows with mild aftertreatment warnings before a bigger issue. Common failure points are the wiring harness near the sensor head, where moisture gets in.
The DEF dosing valve (doser) is typically mounted in the exhaust aftertreatment piping upstream of the SCR catalyst (often on the decomposition pipe). Look for a small injector-style body with an electrical connector and a DEF line attached.
Tip: White crust around DEF fittings or injector body usually indicates crystallization.
If your tool can’t run actuator tests, you can still diagnose a lot with careful inspection + repeatability testing.
Warning aftertreatment faults often start as an emissions warning, then progress to torque derate if the fault remains active over multiple drive cycles. If ignored, many platforms can eventually enter inducement and severe speed limiting.
Exact behavior depends on ECM calibration and which companion faults are active.
If the fault repeats after clearing, diagnose it as ACTIVE. Many faults escalate into derate when ignored.
Mechanic community notes for SPN 3226 FMI 4
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