Code PO420 PASS/NO PASS smog test??.... FIXED and passed smog in socal

Honda pc · Oct 17, 2021

update 7/06/26.... well... I finally fixed it for good....meanwhile I was limping it along with various driving test at different speeds and 420 went away and got test passed barely.....but later next registration...code 420 would not go away and had to dig in hard....I replaced the front catalytic converter Eastern brand from rock auto and it was supposedly CA carb legal and my smog test passed and got my tags...It wasn't that hard to do....just unbolt the flanges and put in the new catalytic converter plus new gasket and done...part was like $400-500.... dealer price was crazy high,,,more the wha the car was worth.

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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19963.4lsr5 · Oct 17, 2021

Code PO420 PASS/NO PASS smog test??

Do you have any exhaust leaks?

Are you topping off the antifreeze regularly?

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Devbot · Oct 17, 2021

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.

Honda pc · Oct 17, 2021

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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Pyral · Oct 17, 2021

19963.4lsr5 said:
Do you have any exhaust leaks?

Are you topping off the antifreeze regularly?

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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.

19963.4lsr5 · Oct 17, 2021

Code PO420 PASS/NO PASS smog test??

Pyral said:
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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Endlessblockades · Oct 18, 2021

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.

JoeT4R · Oct 19, 2021

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

Honda pc · Jul 7, 2026

update 7/06/26.... well... I finally fixed it for good....meanwhile I was limping it along with various driving test at different speeds and 420 went away and got test passed barely.....but later next registration...code 420 would not go away and had to dig in hard....I replaced the front catalytic converter Eastern brand from rock auto and it was supposedly CA carb legal and my smog test passed and got my tags...It wasn't that hard to do....just unbolt the flanges and put in the new catalytic converter plus new gasket and done...part was like $400-500.... dealer price was crazy high,,,more the wha the car was worth.

I will see if I can post he torque app ### on the catalytic converter max and measured values...

Code PO420 PASS/NO PASS smog test??.... FIXED and passed smog in socal

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