Unterdruckschlauch des Wastegates abgezogen

The small hose carrying vacuum or pressure to the wastegate actuator, split, perished or off its spigot. It is a trivial part with a large effect, and it is worth checking before anything expensive because the symptoms are identical to a failed actuator or a seized turbocharger.

Which way the fault presents depends on how the system rests. A vacuum-actuated wastegate is held closed by spring and opened by vacuum, so losing the signal leaves it shut — overboost, and a protective power cut. A pressure-actuated one is the reverse: losing the signal leaves it open, giving underboost and a flat engine. Establishing which this vehicle uses tells you what to expect before anything is measured.

These hoses fail from heat and age rather than damage. Silicone and rubber harden over years next to a turbocharger until they crack at the bend or shrink off the spigot — and the split is frequently on the underside or behind something, which is why a quick look misses it.

Check the whole vacuum circuit rather than just this hose, including the reservoir and its non-return valve. A leak anywhere in it produces the same result, and a system that has lost one hose to age usually has others the same age.