SPN 422 FMI 0Aftertreatment
Catalyst Temperature Too Low (Bank 1)
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Overview
Fault Code
SPN 422 / FMI 0
Category
Aftertreatment
Derate Risk
No
Regen Inhibit
No
Inducement Risk
No
Component Location: Exhaust temperature sensor Bank 1 is located on the aftertreatment system, typically mounted on the DOC inlet or DPF inlet pipe, downstream of the turbocharger on the exhaust mid-pipe.
Common Symptoms
- Check engine light illuminated
- Engine derate or power reduction
- DPF regeneration failures or incomplete regens
- Excessive white smoke during regeneration attempts
- Poor fuel economy
- Extended warm-up periods before normal operation
Root Causes
- DPF or DOC catalyst substrate damaged, melted, or failed (loss of thermal mass/efficiency) Exhaust system air leak upstream of catalyst allowing cold air ingestion and heat loss Faulty or stuck-open exhaust temperature sensor providing erroneous low readings (FMI 0 vs actual temp) Excessive idle time or light-load operation preventing catalyst light-off temperature Failed or degraded DOC (Diesel Oxidation Catalyst) unable to generate exothermic heat during regen Clogged or malfunctioning fuel doser/7th injector not delivering HC for DOC exotherm EGR system stuck open causing excessive exhaust dilution and temperature reduction Turbocharger underperformance reducing exhaust energy and catalyst inlet temperatures Coolant or oil contamination of catalyst substrate reducing catalytic efficiency Harness or connector fault causing false low-temperature signal from exhaust temp sensor
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Derate Behavior
Power Derate
No
Mileage / Regen Notes
SPN 422 FMI 0 (catalyst temperature too low) will inhibit or abort active DPF regeneration. ECM requires catalyst inlet temperature above a minimum threshold (typically 250–300°C DOC inlet) before initiating or continuing regen. Persistent low catalyst temperature will result in DPF soot accumulation, regen inhibit, and eventual DPF high restriction faults. Stationary regen may be disabled until fault is resolved. Extended low-temperature operation risks uncontrolled regen or DPF damage.
System Interactions
SPN 422 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.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 422 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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