Diagnostic Overview
CAT fault code SPN4124 is best normalized as SPN 412 FMI 4. FMI 4 usually indicates EGR temperature circuit voltage below normal or shorted low. The ECM may see an unrealistically low temperature signal and adjust EGR or protection logic incorrectly. This is usually a sensor circuit diagnosis, though the final component name should always be confirmed in Cat ET.
Likely Causes
Short to ground; failed EGR temperature sensor; damaged wiring; poor connector contact; moisture intrusion; low reference voltage.
What To Check First
Compare the temperature reading to ambient when cold, inspect the connector, test signal and ground circuits, and repair wiring before replacing larger EGR components.