SPN 412 FMI 4 — EGR Temperature Circuit Voltage Below Normal
EGRT low-voltage codes require short isolation and multi-temperature correlation. False EGR overtemperature conclusions from shorted sensors are the most documented misdiagnosis — a shorted EGRT circuit produces a false hot reading that can trigger unnecessary cooler replacement.
Fault Overview
SPN 412 FMI 4 indicates the EGR temperature sensor circuit voltage is below the normal minimum — a short circuit or low-signal class fault. Signal short to ground, failed sensor with internal short, insulation damage, and moisture bridging are the primary causes. False EGR overtemperature conclusions from shorted sensors is the most documented misdiagnosis.
Engine & Platform Coverage
Possible Causes
- 1.Signal wire short to ground at chafe points near EGR cooler clamps and brackets
- 2.Failed EGRT sensor with internal short to ground
- 3.Insulation damage from heat exposure near EGR cooler
- 4.Moisture bridging at connector body near EGR plumbing
Wiring & Circuit Notes
Inspect chafe points near EGR cooler clamps and brackets — these are the highest-probability short locations for the EGRT circuit. Check for moisture in the connector body. Correlate with physical conditions after the circuit is proven healthy.
DVOM Specs & Sensor Tests
Near-zero or irrational low signal with the sensor connected confirms a short. Disconnect the sensor and recheck — if the short clears, the sensor is internally shorted. After the circuit is repaired, correlate the EGRT reading against ECT and exhaust temperatures to confirm the reading is plausible. Evaluate cooler efficiency only if the temperature is truly high after sensing 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 EGRT 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 cooler clamps and bracket chafe points
- 5Repair harness short at identified location
- 6Correlate EGRT against ECT and exhaust temps after repair — evaluate cooler only if truly high
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
4 steps locked- 1Disconnect EGRT 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 cooler clamps and bracket chafe points
- 5Repair harness short at identified location
- 6Correlate EGRT against ECT and exhaust temps after repair — evaluate cooler only if truly high
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 412 FMI 4 requires short isolation and multi-temperature correlation. The cooler path is only indicated after the sensing circuit is proven valid and the temperature is truly high.