SPN 421 FMI 0EmissionsMedium
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
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DVOM Testing — in guided session
Connector Pinout — in guided session
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Overview
Fault Code
SPN 421 / FMI 0
Category
Emissions
Severity
Medium
Regen Inhibit
No
Inducement Risk
No
Component Location: Warm-up catalyst (close-coupled TWC) is located directly after the exhaust manifold, upstream of the main underfloor catalytic converter.
Common Symptoms
- MIL (check engine light) illuminated
- Failed OBD-II emissions inspection
- Possible slight increase in fuel consumption
- OBD-II readiness monitor showing catalyst monitor not ready
- No noticeable driveability symptoms in most cases
- DTC stored as confirmed or pending fault
Root Causes
- Degraded or aged warm-up catalyst (TWC) substrate losing conversion efficiency Exhaust leaks upstream of warm-up catalyst skewing oxygen sensor readings Faulty or lazy upstream or downstream oxygen/lambda sensor providing false efficiency calculation Oil consumption fouling catalyst substrate reducing conversion capacity Coolant intrusion into exhaust contaminating catalyst substrate
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System Interactions
SPN 421 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 421 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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