Diagnostic Overview
CAT fault code SPN4115 is best interpreted as SPN 411 FMI 5. FMI 5 usually means current below normal or open circuit on the monitored EGR-related signal or component. If the ECM cannot read or control the EGR pressure/flow circuit properly, it may disable EGR operation and cause emissions, soot, or derate complaints. Diagnosis should begin with circuit integrity rather than replacing mechanical EGR parts.
Likely Causes
Open circuit; disconnected sensor; broken wire; failed EGR pressure sensor; poor connector contact; corroded pins; missing power or ground.
What To Check First
Check the EGR pressure sensor or related component connector, verify supply voltage and ground, test continuity, and confirm the exact component in Cat ET.