Detailed guide for SPN 4342 FMI 4
Back to top ↑What SPN 4342 FMI 4 means
SPN 4342 identifies the aftertreatment DEF line heater 2 circuit on documented applications.
FMI 4 indicates voltage below normal or a circuit shorted toward a low source.
What you may notice
The DEF heating system may not operate correctly.
In cold weather, the affected DEF line may not thaw as expected, which can lead to additional SCR or inducement-related faults depending on the engine calibration.
What to check first
Inspect the line-heater connector and wiring for chafing, corrosion, water intrusion or a short to ground.
Test circuit voltage and continuity using the OEM wiring diagram.
Check related DEF heater faults because multiple heaters can share power or control circuits.
Important
Do not replace the heater until the harness and connectors have been tested. Wiring faults are a common cause of FMI 4 conditions.
The exact location and circuit configuration depend on the engine and vehicle manufacturer.
Mechanic community notes for SPN 4342 FMI 4
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