My Impreza (269k EJ22 - 5 Speed) has intermittently hesitated especially on those damper/snowier days. Felt like the plug wires were due for replacement. Since it was so mild I didn't bother to replace them until yesterday. My wife was driving it home and it stalled out on an intersection. Started back up, but the issue seemed to be getting worse. I swapped the plug wires but got no change.
Today we took it to work. It was pretty goofy on the way. The power would lag and surge and lag and surge. Sometimes rhythmically and sometimes random. On the way home it stalled on me once. I played with it a bit. If I put the clutch in and coasted, it would stall out, then re-engage the clutch and it ran again. It went back and forth from bucking all over the place under load to smoothing right out and being normal.
Stopped at auto-zone to get the CEL code pulled. Code is P0420. CEL has been on for over a year. I generally ignore them unless the car shows issue.
Would an o2 sensor cause this much bucking and issue with the car? Or is this a blocked cat? Or any thoughts on the next best step?