Your DTC reads: SPN 91 FMI 3

Go to the SPN index. Step 2 (Look up SPN 91): You find "Accelerator Pedal Position 1." Step 3 (Look up FMI 3): You go to the FMI table. FMI 3 = "Voltage Above Normal or Shorted High." Step 4 (The Diagnosis): You don't just guess. The PDF tells you that combination means the ECU sees 5+ volts on the pedal sensor circuit when it should see 0.5-4.5 volts. You now know to look for a short to power.

In the world of heavy-duty vehicle communication, standardization is king. For fleet managers, diesel mechanics, and electrical engineers working with commercial trucks, buses, and agricultural machinery, the SAE J1939 family of standards is the Bible. However, within this family lies a specific, often misunderstood, and highly sought-after document: SAE J193973 .