So my Legacy sat almost 3 months (Jan-Mar). Before than the knock sensor code went on. I had it cleared and it didn't go on until last week. I have the codes read and it said misfire all 4 cylinders and knock sensor. I reset my ECU and put new gas and gas treatment. No codes for the past week. I should be all set with the misfiring right? It was probably bad gas? what about the knock sensor? Should I replace it? is it expansive? And can I do it myself?

The engine code system seems to have a little forgiveness in it, in that if the problem persists, it will need to be present sometimes for a few hundred miles to trigger the engine light to come on again. At least that has been my experience with my catalyst code that I've been wrestling with. If your light doesn't come back on after a few hundred miles of driving, then you are probably in the clear.