Kenworth · Barometric Pressure SensorMedium

Kenworth SPN 108 FMI 3: Diagnosis and Repair Guide

SPN 108 FMI 3 (FMI 3) — Barometric Pressure Sensor - Voltage Above Normal, Or Shorted To High Source

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Quick Answer

Kenworth SPN 108 FMI 3 indicates a voltage issue with the barometric pressure sensor, affecting engine performance. Check wiring, connectors, and sensor integrity for resolution.

Overview: Kenworth SPN 108 FMI 3

The Kenworth SPN 108 FMI 3 fault code is a common issue that signals a problem with the barometric pressure sensor circuit. This sensor plays a crucial role in ensuring the engine receives the correct air-fuel mixture by monitoring atmospheric pressure. A malfunction can lead to poor engine performance and reduced fuel efficiency. Addressing this fault promptly is essential to maintain optimal engine operation. The barometric pressure sensor's voltage being above normal or shorted to a high source can disrupt the engine's ability to adjust to changing atmospheric conditions, potentially leading to increased emissions and drivability issues. In addition, failure to resolve this issue can result in long-term damage to engine components due to persistent incorrect air-fuel ratios.

Repair Snapshot

Time

1 - 1.5 Hours

Difficulty

Beginner

Parts Cost

$50 - $125

Dealer Labor

$250+

Tools Required

  • Multimeter
  • OBD-II scanner
  • Basic hand tools
  • Wire brush
  • Electrical contact cleaner

What SPN 108 FMI 3 Means

The SPN 108 FMI 3 code on a Kenworth truck is triggered by an abnormal voltage reading from the barometric pressure sensor. This issue can lead to improper air-fuel mixture adjustments, affecting engine performance and fuel economy. Diagnosing involves checking the sensor's wiring, voltage output, and physical condition. A malfunction can result in the engine control unit (ECU) receiving incorrect atmospheric pressure data, causing it to miscalculate the necessary adjustments for optimal combustion.

Symptoms Drivers Notice

  • Uneven engine idle, causing fluctuations in RPMs.
  • Noticeable decrease in fuel economy, leading to higher fuel costs.
  • Check engine light illuminated on the dashboard.
  • Reduced engine power and sluggish acceleration.
  • Potential increase in exhaust emissions due to improper combustion.

Most Likely Root Causes

Faulty wiring or connectors

Common

Wiring issues, such as corrosion or poor connections, can lead to incorrect voltage readings. Regular inspection and maintenance can prevent these problems. Ensuring proper electrical contact and insulation is critical to avoid intermittent faults.

Defective barometric pressure sensor

Occasional

The sensor itself may fail due to internal faults or exposure to harsh environmental conditions, necessitating replacement. Extreme temperatures and moisture ingress are common factors that can degrade sensor performance over time.

ECU software issues

Rare

Occasionally, the engine control unit may require a software update to correctly interpret sensor data, although this is less common. Software glitches can lead to misinterpretation of sensor inputs, affecting engine management strategies.

Common Misdiagnoses (Don't Waste Parts)

Replacing the sensor without checking wiring

Why it happens: Technicians may assume the sensor is faulty without verifying electrical connections.

Do this instead: Always inspect and test wiring and connectors before replacing the sensor. Use a multimeter to check for continuity and resistance in the circuit.

Ignoring ECU faults

Why it happens: ECU issues are often overlooked, focusing only on physical components.

Do this instead: Consider ECU diagnostics if sensor and wiring checks do not resolve the issue. Look for any stored codes that might indicate ECU-related problems.

Misinterpreting similar fault codes

Why it happens: Similar symptoms can be caused by different codes, leading to incorrect diagnosis.

Do this instead: Use a comprehensive diagnostic approach to ensure the correct fault code is addressed. Cross-reference symptoms with other potential codes to narrow down the cause.

Master Tech Commentary

Mechanic's Notes

Roadside tip: High voltage readings are often due to a faulty ground connection. In 30% of cases, cleaning and securing the ground resolves the issue. Avoid replacing the sensor before confirming the electrical integrity of the circuit. Check for any updates or technical service bulletins related to the ECU software that might address sensor interpretation issues. Additionally, verify the integrity of the sensor's power supply circuit to rule out any voltage regulation problems.

Step-by-Step Diagnostic Procedure

  1. Step 1: Inspect the barometric pressure sensor for visible damage or disconnections. Look for any cracks or signs of physical wear that could compromise sensor functionality.
  2. Step 2: Check wiring and connectors for signs of corrosion or wear that could cause shorts. Use an electrical contact cleaner to remove any buildup and ensure a solid connection.
  3. Step 3: Use a multimeter to measure the sensor's voltage output; compare against manufacturer specifications. Ensure the readings are consistent with the expected voltage range for the sensor.
  4. Step 4: Ensure the sensor is securely mounted and free from debris or obstructions. A loose sensor can lead to inaccurate readings due to vibrations or misalignment.
  5. Step 5: Clear any fault codes with an OBD-II scanner after repairs to verify resolution. Monitor the system for any recurring codes to ensure the issue is fully resolved.

Live Data & Freeze-Frame Tips

When using live data from an OBD-II scanner, monitor the barometric pressure readings while varying engine loads and speeds. Compare these readings against expected values for the current altitude and weather conditions. Consistent discrepancies may indicate sensor or wiring issues. Pay attention to any sudden spikes or drops in pressure readings, as these can be indicative of intermittent faults or electrical noise in the circuit.

Safety Warnings

  • Always disconnect the battery before inspecting electrical components.
  • Ensure the vehicle is on a stable surface to prevent accidental movement.
  • Wear protective gloves and eyewear when working with electrical systems.
  • Avoid working near hot engine components to prevent burns.
  • Follow all manufacturer guidelines when handling sensitive sensors.

When to Limp In vs Tow

If the vehicle exhibits severe drivability issues, such as stalling or significant power loss, towing to a repair facility is advisable to prevent further damage. Persistent issues could lead to unsafe driving conditions, making professional intervention necessary.

Prevention Tips for Fleets & Owner-Operators

  • Regularly inspect and clean sensor connectors to prevent corrosion.
  • Ensure proper routing and securing of sensor wiring to avoid damage.
  • Perform routine diagnostics to catch potential issues early.
  • Update ECU software as recommended by the manufacturer.
  • Use quality replacement parts to ensure sensor reliability.

Related Fault Codes

SPN 107 FMI 3

This code also relates to pressure sensor issues but affects the manifold absolute pressure sensor.

SPN 111 FMI 4

Indicates a low voltage issue in the barometric pressure sensor, the opposite of SPN 108 FMI 3.

SPN 157 FMI 2

Involves the intake air temperature sensor, which can also impact air-fuel mixture adjustments.

Frequently Asked Questions

Can this issue affect emissions?

Yes, incorrect pressure readings can lead to poor combustion and increased emissions, potentially failing emissions tests. The ECU relies on accurate data to manage the air-fuel mixture, and any discrepancies can result in incomplete combustion.

Is this sensor critical for operation?

While not critical for basic operation, it significantly impacts engine efficiency and performance, making it crucial for optimal functionality. A malfunctioning sensor can lead to suboptimal engine tuning and reduced power output.

Should the sensor be recalibrated?

Recalibration is not typically required unless specified by the manufacturer after replacements. Always follow manufacturer guidelines. In some cases, a reset of the ECU may be necessary to clear adaptive memory.

Can I drive with this fault code active?

Driving with this fault code can lead to reduced performance and efficiency. It's advisable to address the issue promptly to avoid long-term damage. Prolonged operation under these conditions can strain other engine components.

How often should I check the barometric pressure sensor?

Regular inspections during routine maintenance are recommended to ensure the sensor and its connections are in good condition. Periodic checks can prevent unexpected failures and maintain optimal engine performance.