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Fault Code Reference

SPN 3216 FMI 4AftertreatmentMedium

Intake NOx Sensor - Voltage Low

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Overview

Fault Code

SPN 3216 / FMI 4

Category

Aftertreatment

Severity

Medium

Derate Risk

Sometimes

Regen Inhibit

No

Inducement Risk

No

Component Location: The intake NOx sensor (SPN 3216) monitors NOx levels in the exhaust stream BEFORE the SCR catalyst — it is the upstream NOx sensor. On most heavy-duty diesel platforms, it is physically mounted in the exhaust pipe or aftertreatment inlet housing upstream of the DPF and SCR system, typically near the turbocharger outlet or in the hot-side exhaust piping before the aftertreatment assembly. Exact location varies by OEM and vehicle configuration: On Cummins ISX/X15 platforms it is commonly located on the exhaust pipe between the turbo outlet and the DPF inlet. On Detroit DD13/DD15/DD16 platforms it is typically mounted on the aftertreatment device inlet. On PACCAR MX-13 and Volvo D13 platforms location is similar — upstream in the exhaust stream before the aftertreatment assembly. The sensor has a threaded bung mount and looks similar to an O2 or NOx probe with a multi-pin connector and heat-shielded pigtail harness. Always verify exact location using the OEM-specific engine/vehicle diagram, as cab-over vs. conventional configurations and chassis mounting variations will affect exact harness routing and accessibility. Based on: general heavy-duty aftertreatment system architecture, OEM system descriptions, field technician reports.

Common Symptoms

  • Check engine light illuminated. MIL lamp on. Possible power derate under extended operation. Erratic or zero NOx readings on scan tool for intake sensor. May see related SCR efficiency codes if system cannot verify NOx levels. Truck may run normally initially but derate can occur if fault remains active for multiple key cycles. Ammonia ratio calculations affected, potentially causing over or under DEF dosing.

Root Causes

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Derate Behavior

Power Derate

Sometimes

Mileage / Regen Notes

Active regeneration is typically NOT inhibited by this fault alone since it is a circuit-level issue rather than an SCR efficiency failure. However, if the ACM cannot read valid intake NOx values, it may limit DEF dosing accuracy which can lead to secondary SCR efficiency faults that WILL inhibit regen. Monitor for related SPN 4364 or SPN 4094 codes. Parked regen may still be commanded but SCR performance cannot be properly verified.

System Interactions

SPN 3216 FMI 4 interacts with the following systems and may trigger cascading faults if not resolved promptly.

DPF differential pressure monitoring — soot load calculations affected
SCR NOx conversion efficiency — DEF dosing strategy may be altered
Active regen inhibit logic — regen may be blocked or delayed
Engine protection derate — ECM may reduce power to protect components
TSB Reference: No confirmed universal TSBs — verify in OEM service portal. Cummins: Check for ISB/ISX aftertreatment harness chafing bulletins. Detroit: Check for DD13/DD15 NOx sensor connector sealing updates. Paccar: Check for MX-13 sensor harness routing advisories. Always search by VIN for applicable campaigns.

Common Diagnostic Mistakes

Replacing the sensor without verifying wiring continuity and supply voltage first.

Clearing codes without identifying and resolving the root cause — the fault will return.

Ignoring related active faults — cascading faults often share a root cause.

Skipping the connector inspection — corrosion and damaged pins are a leading cause of sensor faults.

Condemning the ECM before exhausting all wiring and component tests.

Replacing the DPF without verifying the regen system is functional — a failed regen injector will reload the new DPF.

Ignoring DEF quality — contaminated DEF causes SCR efficiency faults that look like hardware failures.

Deep-dive diagnostic page: SPN 3216 FMI 4 — Full FSM Sequence & Repair Steps — includes wiring notes, sensor specs, and gated repair procedures.

Authority References

Diagnostic procedures on this page are informed by OEM service literature and industry standards.

EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.Cummins QuickServe OnlineOfficial Cummins service manuals, fault code lookup, and TSB database.Detroit Diesel DiagnoCentralDetroit Diesel diagnostic procedures, DDDL documentation, and service literature.PACCAR MX Engine ServicePACCAR MX-11 and MX-13 engine service documentation and DAVIE diagnostic guides.Volvo/Mack Tech ToolVolvo and Mack engine fault code reference and PTT/Tech Tool diagnostic procedures.ASE — Diesel Truck Technician CertificationNational Institute for Automotive Service Excellence — T2 Diesel Engines certification standards.

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