Primary fault reference: SPN 102 FMI 4 — Full Fault Code Reference — This page covers the step-by-step repair workflow. For the complete fault definition, symptoms, causes, and derate risk, see the primary reference above.
Fix SPN 102 FMI 4 — Boost Pressure Sensor Open Circuit
MAP sensor diagnosis, wiring check, and replacement procedure
Overview
SPN 102 FMI 4 indicates the boost pressure (MAP) sensor signal is below normal or the circuit is open. FMI 4 is an electrical fault — the sensor signal voltage is too low, indicating a short to ground or open circuit. The ECM cannot accurately control boost without a valid MAP sensor signal.
Safety Warnings
- ⚠Disconnect battery before working on ECM wiring harness.
Step-by-Step Repair
Read MAP sensor live data
With key on, engine off, read MAP sensor voltage. Should be approximately 1.5–2.0V (atmospheric pressure). If reading is 0V or very low, the circuit has a fault.
Inspect MAP sensor connector
Disconnect the MAP sensor connector and inspect for bent pins, corrosion, or moisture. Clean with electrical contact cleaner if corroded.
Measure MAP sensor reference voltage
With key on, measure voltage between the 5V reference pin and ground pin at the harness connector (sensor disconnected). Should be 4.75–5.25V.
Check signal wire for short to ground
With sensor disconnected and key off, measure resistance between the signal wire and chassis ground. Should be infinite (open circuit). Low resistance = short to ground.
Replace MAP sensor if wiring is good
If reference voltage is correct and no short to ground is found, replace the MAP sensor. MAP sensors fail internally — the signal circuit can be shorted inside the sensor.
Clear codes and verify
Clear fault codes and verify MAP sensor reading is correct at key on, engine off, and at idle. Boost pressure should increase with engine load.
Verification Checklist
- MAP sensor voltage ~1.5–2.0V at key on, engine off
- Boost pressure increases with engine load
- No active fault codes after repair
Related Fault Codes
- SPN 1636 FMI 2 — MAP Sensor Erratic
- SPN 1127 FMI 16 — Boost Pressure Above Normal