SPN 3490 FMI 4 — Aftertreatment SCR Catalyst Temperature Circuit Voltage Below Normal
SCR catalyst temperature low-voltage codes require short isolation before any catalyst conclusions. False SCR efficiency or false overtemperature conclusions from a shorted catalyst temperature circuit are the most documented consequences.
Fault Overview
SPN 3490 FMI 4 indicates the aftertreatment SCR catalyst temperature sensor circuit voltage is below the normal minimum — a short circuit or low-signal class fault. Harness short, failed sensor with internal short, melted insulation, and moisture bridging are the primary causes.
Engine & Platform Coverage
Possible Causes
- 1.Harness short to ground at heat-zone pinch and melt points near SCR catalyst
- 2.Failed SCR catalyst temperature sensor with internal short to ground
- 3.Melted insulation bridging signal to ground near SCR housing
- 4.Moisture bridging at connector body near SCR catalyst
Wiring & Circuit Notes
Inspect heat-zone pinch and melt points near the SCR catalyst — these are the highest-probability short locations. Check for moisture in the connector body. Compare adjacent EGT sensors after the circuit is repaired.
DVOM Specs & Sensor Tests
Disconnect the sensor and check for short to ground. If the short clears with the sensor disconnected, the sensor is internally shorted. After the circuit is repaired, correlate the SCR catalyst temperature against adjacent sensors to confirm the reading is plausible. SCR efficiency evaluation is only appropriate after the catalyst temperature circuit is proven valid.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
5 steps locked behind Technician Mode- 1Disconnect SCR catalyst temperature sensor connector
- 2Check signal wire for short to ground with sensor disconnected
- 3If short clears with sensor disconnected, replace sensor
- 4If short persists, inspect harness at SCR catalyst heat-zone pinch and melt points
- 5Repair harness short at identified location
- 6Correlate SCR catalyst temperature against adjacent sensors after repair — SCR evaluation only after sensing is valid
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
4 steps locked- 1Disconnect SCR catalyst temperature sensor connector
- 2Check signal wire for short to ground with sensor disconnected
- 3If short clears with sensor disconnected, replace sensor
- 4If short persists, inspect harness at SCR catalyst heat-zone pinch and melt points
- 5Repair harness short at identified location
- 6Correlate SCR catalyst temperature against adjacent sensors after repair — SCR evaluation only after sensing is valid
Unlock full diagnostic steps, DVOM tests, and repair procedures.
Full Diagnostic Access
Verification steps, preventative measures, severity guidance, and legacy comparison are available in Technician Mode.
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Summary
SPN 3490 FMI 4 requires short isolation and multi-EGT correlation. SCR efficiency evaluation is only appropriate after the catalyst temperature circuit is proven valid.