After quite a few months of ignoring a P0136 O2 Sensor Circuit Malfunction I finally replaced the front and rear O2 sensors on the Trunkmonkey Pink Impreza. Three miles into the post-installation test drive the car immediately threw a P0325 Knock Sensor Circuit Malfunction. Go figure. I was being punished for something.

A new knock sensor and liberal cleaning of the block didn't solve the problem. Eight miles into the post-installation test drive the car threw a P0325 again. Cleared the code, went for another test drive, threw a code. This went on for a few days of adjusting torque (17.4 ft-lbs), clearing the code, and taking test drives. I finally ended up giving up and asking for help which gave me some more info but didn't help solve the problem.

I acquired a second new knock sensor and, this time, the ECU threw a code immediately after starting the car. No test drive. Just threw a code every time. In disgust I decided to give up on the knock sensor and I replaced the PCV valve which was on my todo list anyway. Just for shits and grins I cleared the code and went out for a test drive. No code. WTF?

So it appears that for some odd reason replacing the PCV valve may have solved the knock sensor issue. Perhaps my grungy old PCV valve with over a hundred thousand miles on it was causing some sort of flutter or harmonic that the knock sensor was picking up? Who knows; all I know is I successfully got the car inspected the next day.