SPN 108 FMI 3 — Barometric Pressure Circuit Voltage Above Normal
BARO high-voltage codes are sensing-path electrical diagnostics first. Boost and fueling complaints at altitude driven by invalid BARO circuit data — with the ECM replaced instead of the sensor — is the most documented misdiagnosis.
Fault Overview
SPN 108 FMI 3 indicates the barometric pressure sensor circuit voltage is above the valid maximum — an open circuit or high-signal class fault on external sensor systems. Open signal wire, failed external BARO sensor, unplugged connector after cab or electronics service, and wiring damage are the primary causes.
Engine & Platform Coverage
Possible Causes
- 1.Open signal wire from damage after cab or electronics service
- 2.Failed external BARO sensor with internal open element
- 3.Unplugged connector after cab or electronics service
- 4.Wiring damage at sensor location from heat or chafe
- 5.Poor reference or ground at BARO sensor connector
Wiring & Circuit Notes
KOEO BARO should approximate local atmosphere when healthy. Compare BARO PID to local atmospheric pressure — significant deviation with no altitude change supports sensing-path fault. Sensor connector moisture and terminal tension should be checked.
DVOM Specs & Sensor Tests
Compare BARO PID to local atmospheric pressure at KOEO. Open circuit values need electrical proof — check supply, reference, and signal continuity. Replace external sensor after circuit is proven. Internal ECM BARO path only if OEM indicates internal BARO and all external supplies are perfect.
Diagnostic Flow — FSM Sequence
AI Diesel Tech enforces this evidence-gate sequence. Each gate must pass before advancing.
4 steps locked behind Technician Mode- 1Compare BARO PID to local atmospheric pressure at KOEO
- 2Check open-circuit continuity from sensor to ECM
- 3Verify supply and reference at BARO sensor connector
- 4Replace external BARO sensor after circuit is proven
- 5Internal ECM path only if OEM indicates internal BARO and all external proof is perfect
Unlock the full evidence-gated diagnostic sequence, DVOM test steps, and repair procedures in Technician Mode.
Repair Steps
3 steps locked- 1Compare BARO PID to local atmospheric pressure at KOEO
- 2Check open-circuit continuity from sensor to ECM
- 3Verify supply and reference at BARO sensor connector
- 4Replace external BARO sensor after circuit is proven
- 5Internal ECM path only if OEM indicates internal BARO and all external proof is perfect
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 108 FMI 3 is a sensing-path electrical diagnosis. Atmosphere correlation and external circuit proof are required before any ECM conclusions.