Diagnostic Overview
P1515 is an EGR-related PACCAR MX fault involving EGR/air-management related performance or derate-linked control fault. The ECM may be commanding EGR operation but seeing a position, pressure, or flow response that does not match expected engine conditions. This can affect soot production, turbo response, emissions performance, and drivability. On PACCAR MX engines, confirm the exact definition with DAVIE4, the engine serial number, emissions year, and current calibration because code meanings and troubleshooting trees can vary.
Likely Causes
Sticking EGR valve; carbon buildup; failed EGR actuator; plugged EGR cooler; failed EGR pressure or temperature sensor; intake or exhaust leak; wiring/connector issue; related turbo or boost fault.
What To Check First
Compare commanded versus actual EGR position, inspect the EGR valve and cooler for carbon restriction, check sensor wiring and connectors, verify pressure/temperature data, and repair any intake, exhaust, or boost leaks.