What is SPN 3556 FMI 1 & How to Solve It?

What is SPN 3556 FMI 1:

SPN 3556 FMI 1 indicates a fault with the Diesel Exhaust Fluid (DEF) Quality Sensor Circuit - High Voltage. This means the engine control module (ECM) is detecting a higher-than-expected voltage signal from the DEF quality sensor, which monitors the concentration and condition of the DEF to ensure effective emission reduction.

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Why This Happens

Common causes include:

• Faulty or failing DEF quality sensor.
  
• Wiring issues such as shorts or damaged connectors.
  
• Contaminated or degraded DEF fluid.
  
• Faulty ECM or sensor calibration problems.
  
  

How to Fix It

To resolve this fault:

1. Inspect the DEF quality sensor and wiring for damage or corrosion.
   
2. Test the sensor signal and replace the sensor if faulty.
   
3. Replace contaminated DEF fluid if necessary.
   
4. Repair or replace damaged wiring or connectors.
   
5. Clear the fault code and monitor system performance.
   
   

How to Avoid It Before It Happens

Prevent this fault by:

• Using clean, high-quality DEF fluid.
  
• Regularly inspecting the DEF quality sensor and wiring.
  
• Maintaining the DEF system to avoid contamination.
• Use Clue’s Preventive Maintenance to schedule regular DEF system inspections, ensuring the DEF quality sensor and wiring are in optimal condition to prevent high voltage issues.