Diagnostic Overview
Mack fault code SPN4125 generally indicates an EGR temperature sensor circuit current below normal or open-circuit type condition. The ECM may be losing the EGR temperature signal, which can affect EGR cooler monitoring and emissions control.
Likely Causes
Open circuit; unplugged sensor; failed EGR temperature sensor; damaged harness; connector corrosion; poor terminal tension.
What To Check First
Inspect the EGR temperature sensor connector and harness, check circuit continuity, and compare live temperature data with key-on.