Like turbochargers in general, the wastegate actuator is the most common component to get the blame incorrectly for a whole manner of engine problems. Remember, it is only a pneumatic device that opens and shuts the wastegate. Its calibration is very important but either it is or it isn't working! Most of the actuators these days are controlled by an electronic solenoid which in turn gets its input from the ECU.
Independent wastegates were traditionally used but as packaging requirements have become more of an issue, wastegate valves are integrally incorporated into the turbine housing of the turbocharger and so a separate device is required to operate it. The actuator resolves this problem by forcing the wastegate valve to open at a preset boost pressure so allowing some engine exhaust gas to bypass the turbine and thus preventing over-speeding. As the engine speed decreases, the wastegate valve is closed and the actuator piston movement is completed.
The traditional turbocharger actuator is a mechanical pneumatic device that senses boost pressure and opens the wastegate valve once a predetermined pressure is reached. The air pressure is sensed typically from either the inlet manifold or the compressor housing of the turbocharger. Inside the canister of the actuator the main components are a diaphragm, retaining cup and spring. The spring is designed to compress below the diaphragm. These two components are separated by the cup, attached to which is a rod that links to the pivoting wastegate.