Hi.. I have a 2002 toyota 4runner 2WD DRIVE socal car and its up for smog test.. The code 420 comes and goes occasionally last 3 yrs.. Had a code420 come up this 2 months .. So I did a reset and been running on 91 octane last 3 tank fill up.. So far,, its only giving a pending fault code420.. no check light but barely failing ## emission number in torque app.. Max is 128..measures 129.. Fail... But no check engine light,, only pending 420 on torque app..

Can i take it like this to test and will it PASS??
Or should i run some lacquer thinner in to pass if possible. Will it work??..
At idle---------O2 reading wave form is steady.
At 2500rpm.. O2 reading is full rapid sine waves which is failing..

Any ideas or inputs what i should do.

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Do you have any exhaust leaks?

Are you topping off the antifreeze regularly?


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Rear O2 sensors commonly trip P0420, though lacquer thinner would be an even cheaper fix. There's also fuel additives specifically sold as "cat cleaner", never tried them myself but I've seen some with really good reviews in the past. Wouldn't hurt to try regardless, I don't see why you shouldn't give it a whirl. I've seen some people in the past unplug an O2/AF sensor and spray something like B12 Chemtool directly into the cat to clean it out, but I've always found this a little bit sketchy - if excess/unburnt fuel can burn a hole in the honeycomb I don't see why naphtha/toluene/xylol/what-have-you wouldn't do something similar. But that's just my perspective.

Didn't check exhaust leak and no regular topping off colant.. So really no coolant level drops... No head gasket problems.

Also.. Is it the small front catalytic converter or the rear one that gives off code 420..

Going to be doing a catalytic converter temp check pre/post to see if catalytic bad....

Anybody got a detailed easy to troubleshoot this code 420 on 3rd gen 2wd toyota 4runner.

New catalytic converter is crazy $$


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Should an exhaust leak throw a code? I know I have a cracked manifold, yet I've never gotten a light or a code for it.

Exhaust systems have a thing called reversion. It causes outside air to be drawn into the system. It messes with O2 readings.


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I had a P0420 "CAT FAILURE" code a couple weeks before my tags expired with a CA Smog Star test looming. Turned out that I had an exhaust flange leak that I caused myself by swapping the MF Overland kit around too hastily. 2 turns of a bolt and I passed smog and haven't seen the code for 5,000+ miles.

I, too, have experienced the P0420 since owning my 4Runner. I've replaced my upstream A/F ratio sensor, and just recently replaced my rear downstream o2 sensor. Denso p/n #234-4261 is what you'll need. It didn't clear my code, but for $40 it's worth a shot. I'll definitely follow this thread for further updates. Also please advise about your cat temp readings