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Faulty Knock Sensor Help Needed

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Hi, my 93 WRX import has been running a rough idle and hesitation for a while now without any engine light coming on. Last week the engine light finally triggered and using the connectors under the dash I managed to get code 22 out of it which I believe is the knock sensor. 


I took the old sensor out and it has 2 or 3 cracks through the plastic so I can safely assume its pretty terminal. Over the weekend I picked up a 2nd hand knock sensor from a Subaru breakers as I wanted to try and get the job done yesterday. Fitted it but it appears to make no difference, still got rough idle and engine light on the dash. I tried hooking up the diagnostic connectors under the dash again in an attempt to reset the ECU but this was still flashing up as code 22. 


I have checked the voltage on the wire that the knock sensor plugs into and I'm getting 5 volts. When the knock sensor is plugged in I only get about 3 volts from the metal off the bottom of it. 


So my question well questions are is that a normal reading? Should the voltage drop like that or is the sensor I picked up no good? Have I missed something with the ECU reset? Do I need to unhook the Battery for 24 hours or is there an easier and quicker way to do it? 


I am nowhere near an electrician but trying to learn the tricks as I go along so any help, hints, tips, kick in the right direction would be greatly appreciated. 

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