P2599: turbo vanes fail to return to their default open position

The generic P2599 name says position-sensor performance stuck high. The cited GM monitor is more specific: variable-geometry turbocharger vanes fail to reach their default fully open position after the actuator is no longer driven.

A secondary check after driver shutdown

P0046 must already be set. The controller waits more than two seconds after shutting off the driver, allowing the spring to return the mechanism, then checks whether position remains above 25%. The row lists no further debounce once the qualified error exists, with a 6.25 ms task and Type A, one-trip reporting.

The table also requires usable position and actuator-related fault information. It covers the named single- and two-stage VGT DC-motor strategies without proving any particular vehicle application.

What changes the diagnostic sequence

Preserve the initiating P0046 event and the subsequent shutdown-position information. Testing only ordinary commanded vane tracking misses the return-to-default behavior that defines P2599 here. Equally, a position value taken before the two-second return allowance is not the completed test.

The strategy supports concern about missing default return, but the code alone does not locate a mechanical restriction or justify replacing the entire turbocharger. Verification should follow the specified service procedure and confirm the default-return assessment with trustworthy position feedback, rather than repeatedly forcing the actuator by hand.

Source and application limits

This explanation covers the cited 2025 GM monitor. Confirm the vehicle and calibration in its service information before applying these limits; inclusion in this file does not establish model compatibility. GM diagnostic parameters, PDF page 1506.