SPN 641 FMI 3 indicates the turbocharger actuator control circuit voltage is above the normal maximum — an open circuit or high-signal class fault on the control side. Open control wire, unplugged actuator connector, failed actuator with internal circuit open, damaged pigtail at the heat shield, and poor supply feed are the primary causes. Actuator replaced while a heat-damaged pigtail remains open is the most documented misdiagnosis.

Inspect actuator connector lock tabs, burnt pins, and brittle insulation near turbo heat. Check for chafe on bracket edges along the actuator pigtail. Confirm connector is fully seated — a partially locked connector can pass visual inspection but fail under vibration.

Confirm supply and control voltage at the actuator connector during a commanded position change. Measure actuator circuit resistance versus OEM specification. Continuity from the driver module or ECM to the actuator must be intact before any actuator is condemned. An open circuit will prevent any commanded response regardless of actuator condition.