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Fault Code Reference

SPN 3237 FMI 0AftertreatmentInducement Risk

NOx Sensor 2 - Data Valid But Above Normal Operating Range

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Overview

Fault Code

SPN 3237 / FMI 0

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: NOx Sensor 2 is mounted in the exhaust pipe downstream of the SCR catalyst, typically on the outlet cone or tailpipe section near the rear of the vehicle.

Common Symptoms

  • High NOx readings displayed on scan tool for downstream NOx sensor
  • Amber check engine light illuminated
  • DEF consumption higher than normal
  • Excessive DEF dosing/injector clicking
  • Failed emissions test or NOx compliance fault
  • Poor drivability or hesitation under load

Root Causes

  • Failed or degraded downstream NOx sensor (sensor 2) reporting abnormally high NOx values post-SCR
  • SCR catalyst degradation or poisoning causing poor NOx conversion efficiency
  • Insufficient or contaminated DEF (diesel exhaust fluid) quality reducing SCR effectiveness
  • DEF dosing system malfunction - clogged or faulty DEF injector reducing urea delivery
  • DEF pump failure causing low or no reagent delivery to SCR catalyst
  • Exhaust system air leak between SCR catalyst and downstream NOx sensor
  • NOx sensor 2 connector corrosion or moisture intrusion causing erroneous high readings
  • SCR catalyst substrate damage or cracking reducing active surface area
  • DEF supply line restriction or freeze causing dosing interruption
  • ACM/ECM software or calibration issue causing misinterpretation of sensor signal

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Derate Behavior

Power Derate

Yes

SCR Inducement Risk

If left unresolved, this fault may trigger SCR inducement — a progressive derate enforced by the ECM to ensure emissions compliance.

System Interactions

SPN 3237 FMI 0 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

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 3237 FMI 0 — 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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