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Pressurised air escaping from the ducting between the compressor and the engine, so the boost the turbocharger genuinely produces never reaches the cylinders. The control module sees a target it cannot meet and blames the part it commands — the wastegate, the vane actuator, the turbocharger itself — which is why a leak so often ends with an expensive part replaced and the fault still present.

It is frequently audible before it is measurable. A whistle or a chuff under load that is absent at idle is the leak opening as pressure rises, and it is worth listening for before anything is connected.

The tell in the data is the disagreement between two measurements. A turbocharger working into a leak still spins hard and its own speed or compressor-outlet reading looks healthy, while the manifold pressure downstream falls short. Two boost readings that disagree with each other locate the leak between them, and that is more useful than either reading alone.

Where it leaks is predictable. A charge hose relaxing off its stub under pressure, a clamp that was never retorqued after a service, a split on the underside of a bend, a perished intercooler end-tank seal, or a sensor grommet not seated. Pressure-test the system cold rather than hunting it running — a leak that only opens under boost is silent at idle, which is precisely why it survives an inspection.