Diagnostic Overview
P1109 is listed as a PACCAR MX derate-related fault. These P-codes can be associated with engine protection, air-management, emissions, or calibration-specific conditions that cause the ECM to reduce power. The code should be interpreted with the complete fault list, not in isolation. 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
Sensor rationality issue; intake or boost control problem; aftertreatment-related fault; wiring problem; failed actuator or sensor; calibration-specific inducement condition; root fault elsewhere in the active code list.
What To Check First
Check the complete active/inactive code set, review freeze-frame data, verify sensor readings for plausibility, inspect wiring and connectors, and identify the root code that triggered the derate.