SPN 3216 FMI 4: NOx sensor signal too low (inlet/upstream) – wiring or sensor failure. For Volvo. Symptoms (3), causes (3), and fixes (3). What to check first to prevent derate and downtime.
| Code | SPN 3216 FMI 4 |
|---|---|
| Severity | Warning |
| Applies to | Volvo |
| Can I drive? | Usually yes for a short distance, but fix it soon. Warning faults like SPN 3216 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 3216 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).
This page is written for Volvo, but the diagnostic logic is similar across many OEMs.
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 shows up constantly on highway trucks because the upstream NOx sensor lives in a miserable spot. If the connector is corroded, a brand-new sensor won’t save you.\n\nQuick win: unplug, inspect, clean, dry, and make sure pins actually grip.
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 3216 FMI 4
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