SPN 173 FMI 4 — Exhaust Gas Temperature Circuit Voltage Below Normal
EGT low-voltage codes need short isolation before thermal conclusions. False overtemp derates from shorted EGT circuits is the most documented consequence — the engine derated for a temperature that was never real.
Fault Overview
SPN 173 FMI 4 indicates the exhaust gas 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 in heat zones, and moisture intrusion are the primary causes. A shorted EGT circuit can produce false overtemp derates.
Engine & Platform Coverage
Possible Causes
- 1.Harness short at melt and pinch points in heat zones around manifold, turbo, and AT
- 2.Failed EGT sensor with internal short to ground
- 3.Melted insulation bridging signal wire to ground near heat sources
- 4.Moisture intrusion at connector body near exhaust components
- 5.Clamp abrasion through heat sleeve insulation
Wiring & Circuit Notes
Inspect melt and pinch points in heat zones around the manifold, turbo, and AT package — these are the highest-probability short locations for EGT circuits. Check for moisture in the connector body. Avoid repeated command into a shorted circuit.
DVOM Specs & Sensor Tests
Disconnect the 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 pinch points in heat zones. After repair, compare neighboring EGT values to confirm the repaired sensor is tracking plausibly. Evaluate true overtemp only after sensing is valid.
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 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 heat-zone melt and pinch points
- 5Repair harness short at identified location
- 6Compare neighboring EGT values after repair to confirm plausible tracking
- 7Evaluate true overtemp 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 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 heat-zone melt and pinch points
- 5Repair harness short at identified location
- 6Compare neighboring EGT values after repair to confirm plausible tracking
- 7Evaluate true overtemp 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
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 173 FMI 4 requires heat-zone short isolation before any thermal or AT conclusions. The EGT circuit must be proven valid before overtemp is accepted as real.