SPN 3490 FMI 3 — Aftertreatment SCR Catalyst Temperature Circuit Voltage Above Normal
SCR catalyst temperature high-voltage codes are open-circuit electrical diagnostics first. SCR catalyst replaced because the temperature circuit was open and the efficiency monitor went irrational is the most documented misdiagnosis — the catalyst was not failing; the sensor circuit was open.
Fault Overview
SPN 3490 FMI 3 indicates the aftertreatment SCR catalyst temperature sensor circuit voltage is above the normal maximum — an open circuit or high-signal class fault. Open circuit, failed sensor, unplugged connector after AT service, and heat-zone harness damage are the primary causes. SCR catalyst replaced because temperature circuit was open and efficiency monitor went irrational is the most documented misdiagnosis.
Engine & Platform Coverage
Possible Causes
- 1.Open circuit from heat-zone harness damage at SCR catalyst temperature sensor
- 2.Failed SCR catalyst temperature sensor with internal open element
- 3.Unplugged connector after AT service
- 4.Heat-zone harness damage at AT harness clamps near SCR catalyst
Wiring & Circuit Notes
Inspect the SCR catalyst temperature sensor connector and AT harness near the SCR module. Confirm connector is fully locked after any recent AT service. Check for heat-zone harness damage along the harness run near the SCR catalyst.
DVOM Specs & Sensor Tests
An open SCR catalyst temperature circuit often reads an extreme cold value. Correlate with adjacent EGT sensors once the circuit is repaired to confirm the reading is plausible. SCR efficiency evaluation is only appropriate after the catalyst temperature circuit is proven valid — an open circuit produces irrational efficiency monitor data.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
4 steps locked behind Technician Mode- 1Inspect SCR catalyst temperature sensor connector — confirm fully locked after any recent AT service
- 2Perform circuit open checks — continuity from sensor to ECM
- 3Replace SCR catalyst temperature sensor after circuit is proven clean
- 4Correlate SCR catalyst temperature against adjacent EGT sensors after repair
- 5SCR efficiency evaluation only after sensing is confirmed valid
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Inspect SCR catalyst temperature sensor connector — confirm fully locked after any recent AT service
- 2Perform circuit open checks — continuity from sensor to ECM
- 3Replace SCR catalyst temperature sensor after circuit is proven clean
- 4Correlate SCR catalyst temperature against adjacent EGT sensors after repair
- 5SCR efficiency evaluation only after sensing is confirmed 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 3 is an open-circuit SCR catalyst temperature diagnosis. The temperature circuit must be proven valid before any SCR efficiency or catalyst conclusions are drawn.