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Mercedes-Benz SPN 102 FMI 3: Boost Pressure Sensor Circuit Shorted High

SPN 102 FMI 3 (FMI 3) — Intake Manifold 1 Pressure - Voltage Above Normal, Circuit Shorted High

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Quick Answer

SPN 102 FMI 3 means the boost pressure sensor signal is reading an electrically impossible high voltage, almost always from a wiring short rather than real overboost.

Overview: Mercedes-Benz SPN 102 FMI 3

This is a wiring diagnosis, not a sensor replacement. Check for chafing near the turbo before touching the part.

Repair Snapshot

Time

1 - 2 Hours

Difficulty

Intermediate

Parts Cost

$80 - $220

Dealer Labor

$350+

Tools Required

  • Multimeter
  • Wiring Diagram
  • Electrical Contact Cleaner

What SPN 102 FMI 3 Means

SPN 102 FMI 3 on Mercedes-Benz OM-series engines means the boost pressure sensor's signal circuit is reading an electrically impossible high voltage, pointing to a short to the sensor's supply wire in the harness rather than a genuine overboost condition.

Symptoms Drivers Notice

  • Possible reduced power as the ECU defaults to a conservative fueling strategy
  • Amber check-engine lamp
  • Truck may feel sluggish without matching turbo noise
  • Intermittent if caused by vibration-related chafing

Most Likely Root Causes

Chafed harness shorting to supply wire

Common

Vibration against turbo heat shields wears through insulation.

Corroded connector

Common

Corrosion can bridge adjacent pins.

Failed sensor internally shorted

Occasional

Less common than wiring but occurs with age.

Common Misdiagnoses (Don't Waste Parts)

Replacing the sensor first

Why it happens: It's the obvious named component.

Do this instead: Backprobe before replacing to confirm fault location.

Assuming a real overboost condition

Why it happens: The code mentions pressure.

Do this instead: FMI 3 means an electrical fault, not a real reading.

Master Tech Commentary

Mechanic's Notes

Shop tip: chafe damage against the turbo heat shield is the top cause on OM-series engines. Check there before ordering a sensor.

Step-by-Step Diagnostic Procedure

  1. Step 1: Locate the boost sensor on the intake manifold or CAC outlet.
  2. Step 2: Backprobe the signal wire with key on, engine off - a reading pegged near supply voltage confirms the short.
  3. Step 3: Trace the harness near the turbo or CAC piping for chafe points.
  4. Step 4: Check for corrosion at the connector.
  5. Step 5: Replace the sensor only after wiring has checked out clean.

Live Data & Freeze-Frame Tips

With the sensor unplugged, check if voltage is still high - the short is upstream in the harness if so.

Safety Warnings

  • Let the turbo cool before probing nearby wiring.

When to Limp In vs Tow

Not required - the truck remains driveable with reduced performance.

Prevention Tips for Fleets & Owner-Operators

  • Route repair harnesses away from turbo heat shields.
  • Inspect connector seals annually.

Related Fault Codes

SPN 102 FMI 4 — Boost Pressure Sensor Circuit

Same circuit shorted low instead of high.

Frequently Asked Questions

Does this mean my turbo is overboosting?

No, FMI 3 indicates an electrical fault, not a genuine overboost.

Can I keep driving?

Yes short-term, though performance may be reduced.

Is this expensive?

Usually not - a wiring repair costs far less than a sensor.

Will a new sensor fix it if wiring is the cause?

No, the code will return.

What tool do I need?

A basic multimeter and wiring diagram.

More Mercedes-Benz codes on this system

All Mercedes-Benz fault codes

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