SPN 108 FMI 4 – Barometric Pressure Circuit Failed Low Fault Code (Symptoms, Causes & Fix)
The SPN 108 FMI 4 fault code indicates a 'Barometric Pressure Circuit Failed Low' condition in a diesel engine. This diagnostic trouble code suggests that the Engine Control Module (ECM) is receiving a signal from the barometric pressure sensor that is lower than expected. The ECM relies on accurate barometric pressure readings to adjust fuel delivery and engine timing for optimal performance.
What SPN 108 FMI 4 Means
When the SPN 108 FMI 4 fault code is active, it means the barometric pressure sensor is reporting a voltage that is below the acceptable range. This sensor is crucial for measuring atmospheric pressure, which the ECM uses to calculate air density and adjust engine parameters accordingly. A failed low signal can lead to incorrect air-fuel mixture calculations, affecting engine efficiency and emissions.
Common Causes of This Fault Code
- Failed barometric pressure sensor
- Wiring harness damage
- Connector corrosion
- ECM signal problems
- Voltage reference faults
Symptoms Drivers May Notice
Drivers may experience symptoms such as reduced engine performance, poor fuel economy, and potentially an illuminated check engine light. In some cases, the engine may enter a derate mode to protect itself from damage due to incorrect air-fuel mixture calculations.
How Technicians Diagnose This Fault
Technicians begin diagnosing the SPN 108 FMI 4 fault code with a visual inspection of the wiring harness and connectors for signs of damage or corrosion. Next, they verify the sensor's power supply and ground circuits using a multimeter. Checking the 5-volt reference circuit is crucial to ensure proper sensor operation. Technicians then test the barometric pressure sensor's output voltage and compare it with a known good sensor or barometer. Finally, ECM data is verified using diagnostic software to ensure accurate readings.
Common Misdiagnosis
A common misdiagnosis involves replacing the barometric pressure sensor without first verifying the integrity of the wiring and connectors. Technicians may also overlook corroded connectors or assume an ECM fault without proper testing, leading to unnecessary part replacements.
Can You Keep Driving?
While it may be possible to continue driving with the SPN 108 FMI 4 fault code active, it is not advisable. The fault can lead to engine derate or protection mode, reducing performance and potentially causing further issues. Addressing the fault promptly is recommended to maintain optimal engine operation.
Mechanic's Diagnostic Tip
Always verify the integrity of the wiring and connectors before replacing the barometric pressure sensor. Corrosion or damage in these areas is a common cause of this fault code and can often be resolved without replacing the sensor itself.
Frequently Asked Questions
Q: What tools are needed to diagnose SPN 108 FMI 4?
A: A multimeter and diagnostic software are essential for testing the sensor and verifying ECM data.
Q: Can a faulty barometric pressure sensor affect fuel economy?
A: Yes, a faulty sensor can lead to incorrect air-fuel mixture calculations, impacting fuel economy.
Q: Is it necessary to replace the ECM if this fault code is active?
A: Not necessarily. Proper testing of the sensor and wiring should be conducted before considering ECM replacement.
