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Problems with catalytic converter

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?


I’d replace the upstream O2 sensor before doing anything else.

It’s possible to look at the signals coming from the O2 sensors with a graphing scanner or oscilloscope and tell what’s really going on, instead of blindly replacing parts.

It could be the sensor, the converter, both, or something else.

The same cause that fouled your #1 plug may also have destroyed your upstream oxygen sensor and your catalytic converter. The root question is why was the #1 plug fouled. Are you burning oil? Are these the original plugs? Was the fouling due to oil? Carbon?

In short, if the plug was misfiring it would be allowing carbon to coat the upstream sensor and the catalyst. Both need to maintain contact with the exhaust to do their jobs. If the cause of the misfiring was fouling, you need to determine what the electrodes are fouled with. You may then also need to replace your converter and/or the upstream sensor. As Circuitsmith suggested, a scope can tell a agood tech lots…including if you have an ignition problem causing the erratic firing in the 1st cylinder.

The #1plug was not firing because the wire had been disconnected. I should have mentioned this on my original post.

Is there a way to test an O2 sensor without a scope? I do have a digital multimeter.

Thanks for the help thus far.