Diagnostic Overview
Mack fault code SPN4120 generally indicates an EGR temperature value above the expected range. High EGR temperature may point to cooling problems, cooler restriction, or sensor error. The code should be checked alongside coolant temperature and EGR cooler efficiency data.
Likely Causes
Restricted EGR cooler; low coolant; poor coolant flow; stuck bypass; faulty EGR temperature sensor; wiring issue.
What To Check First
Check coolant level and flow, compare EGR temperature data, inspect the EGR cooler, and verify the sensor circuit.