Catalytic converter 420 code

1996 4runner here (187,500miles), with weld in after market cat (on when purchased, with less than 1000 mile on it). started throwing the 420 code beginning of last summer. Replaced both O2 sensors and same results. Was coming on after every 30-40 miles after resetting, replaced wires, plugs, cleaned throttle body started coming on ever 120 miles or so. Now in the middle of winter it has gone 450 miles and still has not come back on?? Does the winter fuel blends in Georgia, or cold more dense air make a difference on cat performance? Still plan to replace it with a direct fit with a warranty but curious if I am overlooking something.


Need to read the O2 sensor signals with a scope or scanner that can graph real time data.
I like to measure what’s really going on instead of guessing.

If you don’t need an emissions test, save your money…

My guess is that the welded-in aftermarket converter is just performing its function marginally well enough to satisfy the ECU. In those cases I have seen a code come & go. Generally I’ve seen it on a converter that was on the cusp of failing once and for all, but it would make sense in your case also.

Welding on a generic cat is always a crap-shoot. The capacity of a converter to process oxides of nitrogen varies, and in fact there are many on the market now that are actually designed as “secondary” converters, installed in the midpipe jusr fore of the muffler, and unmonitored. My car has one, and they’ve become common.

I’ll bet that the new direct fit unit will solve the problem permanently. Post back and let us know.

will go with a new direct fit before my emissions test in November. But do you think thank colder weather and winter blend fuel is making it come on less frequent?

Not really. I think it’s coincidence.

Like circuitsmith said, someone should check out the actual O2 sensor readings to get a bead on what is happening. The only think the P0420 really means is that the computer sees signals from the downstream (postcat) sensor that look too similar to the ones from the upstream (precat) sensor. One can only infer that the cause of this is poor cat performance after eliminating any other reason that that these readings could occur.

So someone who knows something should check out the sensor signals, check the circuits for the sensors, and check it for exhaust leaks - maybe especially around those weld points on the replacement cat. There’s probably other stuff.

I don’t know enough to comment on the fuel question.