SPN 108 FMI 4 — Barometric Pressure Circuit Voltage Below Normal
BARO low-voltage codes need short and bus isolation before sensor conclusions. False altitude compensation from shorted BARO data affecting boost and fueling is the most documented consequence.
Fault Overview
SPN 108 FMI 4 indicates the barometric pressure sensor circuit voltage is below the valid minimum — a short circuit or low-signal class fault. Signal short to ground, failed BARO sensor, crushed harness, moisture bridging, and shared reference supply collapse are the primary causes.
Engine & Platform Coverage
Possible Causes
- 1.Signal short to ground at sensor location or harness pinch points
- 2.Failed BARO sensor with internal short to ground
- 3.Crushed harness near sensor mounting location
- 4.Moisture bridging at BARO sensor connector
- 5.Shared reference supply collapse from another shorted sensor on the same bus
Wiring & Circuit Notes
Check shared reference bus voltage before condemning the BARO sensor — a collapsed 5V bus from another shorted sensor on the same supply will produce a BARO low-voltage code. Disconnect isolation identifies whether the short is in the BARO sensor or the harness.
DVOM Specs & Sensor Tests
Check 5V bus voltage at BARO sensor connector. Disconnect BARO sensor and check signal wire for short to ground. If short clears with sensor disconnected, the sensor is internally shorted. If short persists, inspect harness at sensor location and pinch points. If bus voltage is low with BARO disconnected, identify the shorted companion sensor on the shared supply.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
6 steps locked behind Technician Mode- 1Check 5V bus voltage at BARO sensor connector
- 2Disconnect BARO sensor connector
- 3Check signal wire for short to ground with sensor disconnected
- 4If short clears with sensor disconnected — replace BARO sensor
- 5If short persists — inspect harness at sensor location and pinch points
- 6If bus voltage is low with BARO disconnected — identify shorted companion sensor
- 7Repair identified fault and verify BARO reads near local atmosphere
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
5 steps locked- 1Check 5V bus voltage at BARO sensor connector
- 2Disconnect BARO sensor connector
- 3Check signal wire for short to ground with sensor disconnected
- 4If short clears with sensor disconnected — replace BARO sensor
- 5If short persists — inspect harness at sensor location and pinch points
- 6If bus voltage is low with BARO disconnected — identify shorted companion sensor
- 7Repair identified fault and verify BARO reads near local atmosphere
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 108 FMI 4 requires bus voltage check and disconnect isolation before sensor conclusions. Shared-supply awareness is required.