Diagnostic Overview
U1158 is a communication-type fault related to the turbo, VGT actuator, or air-management network on the PACCAR MX platform. If the ECM loses reliable communication with the turbo actuator or related controller, boost control can become inaccurate and the engine may derate. 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
CAN wiring fault; actuator power or ground issue; loose connector; water intrusion; failed VGT actuator electronics; low system voltage; harness rub-through; incorrect actuator configuration.
What To Check First
Check battery voltage and grounds, inspect turbo actuator connectors, verify power/ground/data-link circuits, look for related U-codes, and confirm the actuator communicates and calibrates correctly.