SPN 412 FMI 3 — EGR Temperature Circuit Voltage Above Normal
EGRT high-voltage codes are open-circuit electrical diagnostics first. An EGR cooler replaced because the EGRT was open and always read cold is the most documented misdiagnosis — the cooler was not failing; the sensor circuit was open and producing a false cold reading.
Fault Overview
SPN 412 FMI 3 indicates the EGR temperature sensor circuit voltage is above the normal maximum — an open circuit or high-signal class fault. Open signal wire, failed thermistor or thermocouple path, unplugged connector, and harness damage after cooler service are the primary causes. EGR cooler replaced because EGRT was open and always read cold is the most documented misdiagnosis.
Engine & Platform Coverage
Possible Causes
- 1.Open signal wire from heat damage near EGR cooler and valve region
- 2.Failed EGRT sensor — open thermistor or thermocouple element
- 3.Unplugged connector after EGR cooler service
- 4.Brittle heat-zone insulation causing intermittent or permanent open
Wiring & Circuit Notes
Inspect the sensor connector at the EGR cooler and valve region for heat damage and brittle insulation. Confirm connector is fully locked after any recent cooler service. Check for heat-zone insulation damage along the harness run near the cooler.
DVOM Specs & Sensor Tests
An open EGRT circuit often reads an extreme cold value — the ECM interprets the open circuit as a very low temperature. Compare the EGRT reading against ECT and exhaust temperatures for plausibility once the circuit is repaired. The EGR cooler path is only indicated if true thermal evidence remains after the sensing 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- 1Inspect connector at EGR cooler region — confirm fully locked after any recent service
- 2Perform circuit open checks — continuity from sensor to ECM
- 3Perform OEM sensor circuit resistance test
- 4Replace EGRT sensor after circuit is proven clean
- 5Compare EGRT against ECT and exhaust temps after repair for plausibility
- 6EGR cooler path only if true thermal or leak evidence remains 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- 1Inspect connector at EGR cooler region — confirm fully locked after any recent service
- 2Perform circuit open checks — continuity from sensor to ECM
- 3Perform OEM sensor circuit resistance test
- 4Replace EGRT sensor after circuit is proven clean
- 5Compare EGRT against ECT and exhaust temps after repair for plausibility
- 6EGR cooler path only if true thermal or leak evidence remains 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.
Start Diagnosing This Fault Now
AI Diesel Tech walks you through every evidence gate in the correct order. Choose the plan that fits your shop.
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Summary
SPN 412 FMI 3 is an open-circuit EGRT diagnosis. The cooler path is only indicated after the sensing circuit is proven valid and true thermal evidence remains.