What is SPN 791 FMI 2 & How to Solve It?

What is SPN 791 FMI 2:

SPN 791 FMI 2 indicates a fault with the Boost Pressure Sensor Circuit – Voltage Below Normal. This means the engine control module (ECM) is receiving a lower-than-expected voltage signal from the boost pressure sensor, which measures the pressure of the air entering the engine’s intake manifold, essential for optimizing engine performance.

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Why This Happens

Common causes include:

• Faulty or failing boost pressure sensor.
  
• Damaged or corroded wiring/connectors in the sensor circuit.
  
• Leaks in the intake system causing inaccurate pressure readings.
  
• Faulty ECM or sensor calibration issues.
  
  

How to Fix It

To resolve this fault:

1. Inspect the boost pressure sensor and wiring for damage or corrosion.
   
2. Check for leaks or damage in the intake system.
   
3. Test the sensor voltage output and replace if faulty.
   
4. Repair or replace damaged wiring or connectors.
   
5. Clear the fault code and verify proper boost pressure readings.
   
   

How to Avoid It Before It Happens

Prevent this fault by:

• Regularly inspecting the boost pressure sensor and intake system.
  
• Maintaining a sealed intake system free of leaks.
  
• Performing routine diagnostics to detect sensor or wiring issues early.
• Use Clue’s Preventive Maintenance to schedule regular checks on the boost pressure sensor, wiring, and intake system, preventing potential faults before they affect engine performance.