Changing a Subaru Legacy 1999 knock sensor is dead easy. You need only a socket wrench with a long extension, a pair of strong scissors, and thin fingers.

A new sensor will cost around $120 online.

The sensor is located on the engine block just under the air intake manifold (see arrow and zoomed image). You will need to unscrew a bolt to get at the wires, then unscrew the bolt that holds the sensor to the engine block. There is a tie around the knock sensor wire, which I simply cut off with the scissors. Pull out the sensor and then disconnect the harness. Install the new sensor by plugging in the harness and attaching the same bolt that held the old one.

Start the car and you should have all your old power back, plus you saved yourself about $200 relative to getting it done at the dealership.