Good afternoon,

My 1999 Tracker threw a P0420 code last week. I've owned the car about seven years and I have already replaced both upstream and downstream O2 sensors (the car threw codes for both at different times.) So I did a little homework on this forum, watched a few Utube videos, and tested my Catalytic Convertor. A good Catalytic Convertor will do three things. (1) The down stream O2 sensor reading will bounce up and down but in a much narrower range than the upstream O2 sensor. (2) The downstream exhaust temperature will be much higher than the upstream exhaust temperature. This can be measured with an infrared thermometer. (3) The Catalytic Convertor will not make a lot of noise or rattle when the engine is revved up.

My Catalytic Convertor failed on all three of these tests. The live data stream showed the downstream O2 sensor bouncing way too much, the downstream exhaust temperature was almost the same as the upstream exhaust temperature, and the catalytic convertor rattled when the engine was revved up.

The other consideration was I had to have the head redone a couple years ago. The engine was misfiring and redoing the head solved that issue. However, the misfire probably sent fuel into the catalytic convertor where it ignited and gradually destroyed the guts of the catalytic convertor, hence the triple fail on my tests.

I mail ordered the part, and I will update this thread after I've installed the new part.

 

I think you are correct. The dreaded code P0300 will do the same if run too long with the code on, since its sending unburned fuel down the pipe to the converter.