I’ve a 2001 Toyota Echo, it has two O2 sensors, the downstream one was replaced six months ago, the upstream one is still the original part. The car has about 176,000 miles.
A couple of weeks ago during a five hour road trip the engine started missing about three hours into the trip, and continued doing it until I got home. The engine light was on by then, with codes P0300 (random misfiring) and P0301 (misfiring on cylinder #1). The plug on #1 was fouled, I replaced all four of them, reset the light, and the engine was back to running normally.
A day or so later the engine light came on again, this time with code P0420 (catalytic converter not operating as efficiently as possible). If I reset it, it comes back on after about 45 minutes.
However, two days ago I went on another longish road trip, resetting the light before starting, and it did not come on at all during highway driving (about 2.5 hours each way). Once I was off the highway and driving on city streets it came back on.
Does this point to a bad catalytic converter? If so, would the unburned fuel from cylinder #1 have been the culprit? Any other troubleshooting ideas?