Cause
Seized turbocharger switching valve
The valve directing exhaust or charge air between two turbochargers, or between stages of a sequential system, seized in one position. Sequential and twin-scroll arrangements switch flow as speed rises, and the valve is what does it.
Stuck in the low-speed position, the engine builds boost quickly and runs out at the top; stuck in the high-speed position, it is lazy low down and strong once spinning. That asymmetry across the rev range is the signature and no fuelling fault reproduces it.
The valve lives in the exhaust stream on most designs, so carbon on the spindle and corrosion in the bore are what hold it — the same mechanism as every other exhaust-side valve. Disconnect the actuator and check the valve moves freely through its full travel by hand, which separates a seized valve from a failed actuator. Its vacuum or electric actuator is the other half, and testing it separately is what avoids replacing the wrong one.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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