Diagnostic Overview
CAT fault code SPN4123 is best interpreted as SPN 412 FMI 3. This usually indicates the EGR temperature circuit voltage is above normal or shorted high. The ECM may receive an unrealistically high temperature signal, which can affect EGR strategy and engine protection logic. Because FMI 3 is usually electrical, the sensor circuit should be tested before replacing the EGR cooler or other mechanical parts.
Likely Causes
Short to voltage; failed EGR temperature sensor; open ground; damaged wiring; corroded connector; incorrect sensor; water intrusion.
What To Check First
Inspect the EGR temperature sensor connector, check reference voltage and ground, test signal voltage, and compare cold sensor reading to ambient temperature.