SPN 2203 FMI 0Emissions
NOx Sensor Circuit High Bank 1
Scan Tool — in guided session
DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 2203 / FMI 0
Category
Emissions
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Upstream NOx sensor (Bank 1) is mounted in the exhaust pipe before the SCR catalyst, typically near the turbocharger outlet or DPF inlet.
Common Symptoms
- DEF / SCR system fault lamp illuminated on dash
- Active fault code logged for NOx sensor high voltage or out-of-range signal
- Elevated or implausibly high NOx reading reported by ECM on Bank 1 sensor
- Possible derate or engine power reduction in emissions-compliance mode
- SCR efficiency fault or catalyst monitoring code set as secondary code
- NOx sensor signal fixed at high value or non-responsive during key cycle
Root Causes
- Failed or degraded upstream NOx sensor (Bank 1) sending voltage above normal operating range
- NOx sensor connector contamination, corrosion, or moisture intrusion causing signal drift high
- Damaged or shorted sensor harness wiring between NOx sensor and DCU/ECM
- Failed NOx sensor control module or sensor heater circuit fault causing erroneous high reading
- ECM/DCU internal fault or software anomaly misinterpreting NOx sensor signal
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
SPN 2203 FMI 0 will typically inhibit or severely restrict active DPF regeneration events and may place the aftertreatment system into a derate or shutdown protection mode to prevent SCR dosing errors.
System Interactions
SPN 2203 FMI 0 interacts with the following systems and may trigger cascading faults if not resolved promptly.
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.
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OEM System Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 2203 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.
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.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.
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