AI Diesel Tech

Fault Code Reference

SPN 4334 FMI 5AftertreatmentInducement Risk

SCR Outlet NOx Reading - Current Below Normal

Run Full DiagnosisView Related FaultsSee OEM System Guide

Scan Tool — in guided session

DVOM Testing — in guided session

Connector Pinout — in guided session

DieselVision™ Upload photos — DieselVision™ reads diagrams, connectors, and screens with precision.

Overview

Fault Code

SPN 4334 / FMI 5

Category

Aftertreatment

Derate Risk

Yes

Regen Inhibit

No

Inducement Risk

Yes — SCR inducement possible

Component Location: SCR outlet NOx sensor is mounted on the exhaust pipe downstream of the SCR catalyst brick, typically near the rear of the aftertreatment assembly under or behind the cab.

Common Symptoms

  • SCR outlet NOx sensor fault active
  • Check engine light illuminated
  • NOx sensor implausible or out-of-range readings
  • DEF consumption incorrect or not tracking properly
  • SCR efficiency monitoring disabled
  • Unable to verify SCR catalyst performance

Root Causes

  • SCR outlet NOx sensor open circuit or broken signal wire
  • NOx sensor connector corrosion, moisture intrusion, or pin backout
  • Failed SCR outlet NOx sensor (internal open circuit)
  • Damaged or chafed wiring harness between ACM and NOx sensor
  • Blown fuse or open circuit in NOx sensor supply voltage circuit
  • Poor ground connection at NOx sensor or ACM ground reference
  • ACM software or calibration issue causing incorrect current reading
  • NOx sensor heater circuit failure causing sensor not to reach operating temperature

Full step-by-step tests, DVOM guidance, and pinout data are available inside the AI Diesel Tech diagnostic session.

AI walks you through every test — one step at a time. Plans start at $12.95/mo.

Start Guided DiagnosisSee How It Works

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.

Mileage / Regen Notes

An SCR outlet NOx sensor current below normal fault (FMI 5) will typically inhibit closed-loop SCR DEF dosing control and may trigger a derating condition. Active DPF regeneration may be inhibited or forced to passive only if the ACM cannot confirm aftertreatment outlet conditions. Extended fault presence can result in progressive engine derate.

System Interactions

SPN 4334 FMI 5 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 4334 FMI 5 — 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.

Get weekly diesel diagnostic tips

Real-world fault code walkthroughs, wiring tips, and shop-tested fixes — free.

No spam. Unsubscribe anytime.

Get AI-Guided Diagnosis for SPN 4334 FMI 5

The diagnostic engine walks you through every physical test — step by step, on your phone, at the truck. No guessing. No wasted parts.

Run Full DiagnosisView Related FaultsSee OEM System Guide