Exhaust soot over time jams the unison ring (the part that moves the vanes). The actuator tries to move, but the sensor reports “wrong position” for too long → limp mode.
On Renault’s 1.5 dCi (K9K engine), DTC1525F3 specifically points to the turbo variable geometry (VG) actuator position sensor — but not a dead turbo. 90% of the time, the cause is carbon buildup locking the VG vanes . dtc1525f3 renault
Generic scanners say: "Turbocharger Boost Control Position Sensor Circuit Range/Performance." You panic, thinking turbo, actuator, wiring. Exhaust soot over time jams the unison ring