SPN 1172 FMI 4 — Turbocharger Turbine Inlet Temperature Circuit Voltage Below Normal
Turbine inlet temp low-voltage codes need short isolation before thermal conclusions. False overtemp conclusions at the turbo inlet from shorted sensors is the most documented consequence.
Fault Overview
SPN 1172 FMI 4 indicates the turbocharger turbine inlet 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 bridging near the turbine housing, and moisture intrusion are the primary causes.
Engine & Platform Coverage
Possible Causes
- 1.Harness short at melt points and clamp edges around the turbine housing area
- 2.Failed turbine inlet EGT sensor with internal short to ground
- 3.Melted insulation bridging signal wire to ground near turbine housing heat sources
- 4.Moisture intrusion at connector body near turbine housing
- 5.Clamp abrasion through heat sleeve insulation near turbine inlet
Wiring & Circuit Notes
Inspect melt points and clamp edges around the turbine housing area — these are the highest-probability short locations for the turbine inlet EGT circuit. Check for moisture in the connector body.
DVOM Specs & Sensor Tests
Disconnect the turbine inlet EGT sensor and check the signal wire for short to ground. If the short clears with the sensor disconnected, the sensor is internally shorted. If the short persists, inspect the harness at melt and clamp edge points near the turbine housing. After repair, compare neighbor-sensor values to confirm the repaired sensor is tracking plausibly.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Disconnect turbine inlet EGT 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 turbine housing melt and clamp edge points
- 5Repair harness short at identified location
- 6Compare neighbor EGT values after repair to confirm plausible tracking
- 7Evaluate true high EGT only after sensing is valid
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
5 steps locked- 1Disconnect turbine inlet EGT 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 turbine housing melt and clamp edge points
- 5Repair harness short at identified location
- 6Compare neighbor EGT values after repair to confirm plausible tracking
- 7Evaluate true high EGT 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.
Start Diagnosing This Fault Now
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Related Resources
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Common Symptoms
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Summary
SPN 1172 FMI 4 requires heat-zone short isolation before any turbo thermal conclusions. The turbine inlet EGT circuit must be proven valid first.