SPN 106 FMI 4 — Air Inlet Pressure Circuit Voltage Below Normal
Air inlet pressure low-voltage codes need short isolation before restriction decisions. False restriction indications from shorted inlet pressure sensors leading to unnecessary air-path work is the most documented consequence.
Fault Overview
SPN 106 FMI 4 indicates the air inlet pressure sensor circuit voltage is below the valid minimum — a short circuit or low-signal class fault. Signal short to ground, failed 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 housing pinch points
- 2.Failed air inlet pressure sensor with internal short to ground
- 3.Crushed harness at housing clips or adjacent brackets
- 4.Moisture bridging at 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 inlet pressure sensor — a collapsed 5V bus from another shorted sensor on the same supply will produce a low-voltage code. Disconnect isolation identifies whether the short is in the sensor or the harness.
DVOM Specs & Sensor Tests
Check 5V bus voltage at sensor connector. Disconnect 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 housing pinch points. If bus voltage is low with sensor disconnected, identify the shorted companion sensor on the shared supply. Re-evaluate restriction only with valid inlet pressure data.
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 sensor connector
- 2Disconnect sensor connector
- 3Check signal wire for short to ground with sensor disconnected
- 4If short clears with sensor disconnected — replace sensor
- 5If short persists — inspect harness at housing pinch points
- 6If bus voltage is low with sensor disconnected — identify shorted companion sensor
- 7Re-evaluate restriction only with valid inlet pressure data after repair
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 sensor connector
- 2Disconnect sensor connector
- 3Check signal wire for short to ground with sensor disconnected
- 4If short clears with sensor disconnected — replace sensor
- 5If short persists — inspect harness at housing pinch points
- 6If bus voltage is low with sensor disconnected — identify shorted companion sensor
- 7Re-evaluate restriction only with valid inlet pressure data after repair
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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Summary
SPN 106 FMI 4 requires short isolation before restriction decisions. Circuit must be proven valid before any air-path conclusions.