The basics of the N75 Waste Gate Frequency Control valve: how it works alongside the waste gate actuator, the waste gate itself and the turbo. You’ll be able to see how it should function in an inactive and powered state, where it is on the Audi A4 B7 2.0, and how to test and replace. This info is also generally applicable beyond VAG FSI or TSI vehicles.

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The B7 has an internal waste gate (aka, the valve that releases exhaust air into the exhaust is IN the turbo housing on the exhaust side), oppose to an external waste gate that you might find in custom or big turbo set ups. This valve is moved by a rod that is moved by the waste gate actuator. The actuator stays closed by default from its own spring pressure. But when enough boost builds up, the air pressure is forced upon the actuator, moving the rod, opening the waste gate.

But the N75 valve is in the middle of this system. Sometimes, it does nothing, allowing boost to flow through it and act on the waste gate (actuator, rod, valve). But sometimes it needs to block that pressure in order to keep your turbo from spooling above its operating specs or smooth out your driving experience. In that scenario, it recirculates the boost (from the compressor side, not the exhaust side - it's not exhaust gas) back into the turbo intake. No air exits the system unless it's through the exhaust. All air that's sucked into the intake either gets recirculated by the diverter valve, passes through the engine and exits over the turbo turbine and out the exhaust, or passes through the engine and bypasses the turbine and still heads out the exhaust. But it's all accounted for.

Although there can be various issues leading to low boost, if the N75 is stuck in it's default state where the actuator and rod are constantly forcing the waste gate to stay open, the N75 would be responsible. A torn diaphragm on the older style of diverter valves could have the same effect, though. As would a boost leak, a maladjusted waste gate rod, an actuator not responding at the correct pressures - or an electrical issue leading to either the diverter valve or N75.

Part numbers:
N75 Valve: 06F 906 283 F
Hose from compressor: 06D145718M
Hose to turbo intake: 06F145718R
Hose to waste gate actuator: 06F145718Q