what is this knock sensor that everyone is tlaking about and how do i know if mine is bad or not?

welp you could do a search on the subject but suffice it to say if your CEL light comes on and you check the codes and it reads knock sensor ummmmm you need one

On the most basic level, a knock sensor "feels" the harsh harmonic of engine knocking. As with today's engines, not all knocking is detectable to the human ear. Then it retards the timing automatically so you don't have to worry about causing excessive damage.

 

On a smarter car, this is how the ECU knows what octane fuel you are running so you can get the most out of your engine.

 

It looks like a little hockey puck (about 1.5" in diameter) with an electrical connection - and is usually bolted to the block, one per cylinder bank. That's the norm, I don't know how Subaru handles it.

On the EJ22's there is one knock sensor and it is located almost directly under the intake. It's time to replace it when it's cracked. Odds are, if you're on your original, it's cracked and needs to be replaced.

The unit works by generating a voltage when it detects sudden shock to the engine. It is a piezoelectric device. The output of the sensor is tied to the ECU and it will send a code out if there is shock detected. The ECU can determine which cylinder is having trouble since it controls the timing.

If the knock sensor is wack and is reporting knock when there is none then your mileage and performance will suffer. The ECU's job is to interpret the voltage from the ECU to determine whether or not there is knock. Some people replace the knock sensor with a resistor but this action is controversial.