Frequently asked questions about SPN {spn} FMI {fmi}. Covers meaning, symptoms, causes, ECM behavior, electrical tests and step-by-step diagnostic procedures.
Full Diagnostic Guide — SPN 4354 FMI 5 1. What does SPN 4354 FMI 5 mean? SPN 4354 FMI 5 indicates that the Engine Control Module (ECM) has detected a current below normal or an open circuit on the Diesel Exhaust Fluid (DEF) line heater 1 request circuit for aftertreatment system 1. This means the … Read more
Full Diagnostic Guide — SPN 4353 FMI 5 1. What does SPN 4353 FMI 5 mean? SPN 4353 FMI 5 indicates that the Aftertreatment 1 DEF doser heater mode request circuit has detected current below normal or an open circuit condition. The ECM monitors the current draw through the DEF doser heater control circuit and, … Read more
Full Diagnostic Guide — SPN 4349 FMI 7 1. What does SPN 4349 FMI 7 mean? SPN 4349 FMI 7 indicates a failure in the aftertreatment SCR system’s mechanical components to respond correctly to ECM state change commands. This specifically points to a failure in transitioning between dormant, dosing, and protection modes due to mechanical … Read more
Full Diagnostic Guide — SPN 4344 FMI 5 1. What does SPN 4344 FMI 5 mean? SPN 4344 FMI 5 indicates the Aftertreatment 1 DEF Line Heater 3 circuit has detected current below normal or an open circuit. This means the ECM measures an infinite resistance condition on the heater circuit, typically due to a … Read more
Full Diagnostic Guide — SPN 4344 FMI 3 1. What does SPN 4344 FMI 3 mean? SPN 4344 FMI 3 indicates that the ECM has detected a voltage above the acceptable threshold in the Diesel Exhaust Fluid (DEF) line heater 3 circuit. FMI 3 specifically means ‘Voltage Above Normal or Shorted High.’ This fault signals … Read more
Full Diagnostic Guide — SPN 4342 FMI 5 1. What does SPN 4342 FMI 5 mean? SPN 4342 FMI 5 indicates an electrical issue where the ECM detects current below normal or an open circuit condition in the Aftertreatment 1 Diesel Exhaust Fluid Line Heater 2 circuit. This typically signifies that the heater is not … Read more
Full Diagnostic Guide — SPN 4340 FMI 5 1. What does SPN 4340 FMI 5 mean? SPN 4340 FMI 5 indicates a fault in the Diesel Exhaust Fluid (DEF) line heater circuit. Specifically, FMI 5 means the current is below normal or an open circuit has been detected. This prevents the heater from warming the … Read more
Full Diagnostic Guide — SPN 4340 FMI 4 1. What does SPN 4340 FMI 4 mean? SPN 4340 FMI 4 indicates voltage below normal in the diesel exhaust fluid line heater circuit within aftertreatment system 1. The ECM has detected that the electrical voltage supplied to the DEF line heating element has dropped below the … Read more
Full Diagnostic Guide — SPN 4339 FMI 7 1. What does SPN 4339 FMI 7 mean? SPN 4339 FMI 7 indicates a malfunction within the SCR feedback control system, specifically that the mechanical components are not responding correctly to ECM commands. This typically involves a mechanical failure in the SCR dosing valve or a blockage … Read more
Full Diagnostic Guide — SPN 4335 FMI 7 1. What does SPN 4335 FMI 7 mean? SPN 4335 refers to the SCR dosing system air assist absolute pressure sensor. FMI 7 indicates a mechanical response failure—the sensor signal does not match the expected pressure change when the ECM commands reagent injection. The ECM expects a … Read more