Hey guys, I have a 1999 V6 2wd from California. Recently I got a code that meant cleaning out my MAF, I did that and it cleared up the code, but threw a knock sensor code instead so today I got both sensors and harness replaced. After a few miles, P0420 came on. Any suggestions on where to go from here? Both 02's were replaced in the past year or two and I know I've gotten this same code before and have still passed smog by getting it to a smog shop before my cat test completed on the ECU, or at least that's what it seemed like to me. I see that the only test that's pending at the moment is the evap system test, does anyone know if there's a set minimum number of tests that need to be completed for a 99 in Cali for it to be ready to test?

I'm hoping I can complete the evap test before the test that causes the P0420 but unsure how to "force" it if you will.

Yeah its a process to get cleared for evap. Mine gets a 420 everytime I crawl for a couple miles, like clockwork.

I clear it before a smog test, waiting for all but the evap, it never gets ready but in texas we're allowed one to not be ready.

Look up a 420, lots of possible causes besides cats or 02 sensors. Luck.

Yeah, that's the issue I'm running into right now, smog is almost due and I can clear the codes but I'm just trying to get evap to complete before the cat test so I can take it in but it's not wanting to cooperate lol what's weird though is that using OB reader, it's showing p0420 as confirmed but right under it's also showing as pending.

I sound like a broken record, but first check for any exhaust leaks (even well beyond the second Cat and sensor on CA rigs). That's all mine was although my OBDII reader was screaming Catalytic Converter Failure! (2 years ago).

I'll go ahead and buy some seafoam to see if this will identify leaks. While I'm there though, does anyone know if this will work on our 4runners?

How To Fix A Catalytic Converter Check Engine Code With Spark Plug Non Foulers! - YouTube

$4 Catalytic Converter P0420 Fix | 02 Sensor Simulator | Pass Emissions | Oxygen sensor - YouTube

So this morning on the way to work, I checked my OBDII reader and it showed that the evap test had gone through. This was about 60 miles from when I last got gas. I have about a 100 mile commute roundtrip each day so I might be able to get the evap test to complete and then stop at whatever the nearest smog shop is on my way home before the P0420 code comes back up again. It's just going to be a hassle.

I swapped out my alternator today and about 3 miles from home was able to complete the evap cycle, I rushed to try to get it to a smog shop only to have the P0420 code immediately show up as pending so I cleared the codes again. I'll have to try again tomorrow.

Any particular gaskets that are recommened? I see the round ones and also see the ones that have "ears" to the side of the them, not sure which is the one that should go on our 4runners, if there's a real need of one over the other.

Put on a flat pair of shoes and block off the tailpipe. If the engine stalls you are good or have minimal leaks.

The thick round ones are what we use.


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I went through this nightmare recently and I had set an appointment with a muffler shop that was going to figure out which cat was bad. In the meantime, my last ditch effort was to renew the upstream air/fuel sensor and downstream O2 sensor. I was able to finally get rid of the P0420 long enough for the EVAP sensor to reset and I rushed to the smog shop and got it done. Good for another 2 years in the Communist State of California.

Thanks, the O2's on my rig are (fairly) new and didn't resolve the issue when I changed them out last time so don't want to go this route again.

Hey guys, thanks for all your help on this, was out driving with my son today and saw all tests were green except for catalyst, swung right over to smog and it passed! I honestly can't say what would happen if codes were pending as the p0420 code still hasn't popped up after giving such a headache the last few days but I'm hoping this will be the last time I deal with this, I'm thinking of going electric for my next car, God willing

If I understand correctly, as long as the catalyst readiness monitor isn’t set the 420 won’t develop. After it’s set…….


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I'm surprised they did the smog without the catalyst test showing ready. That's usually a fail anywhere in California. That smog tech did you a favor.

Correct, that's why I ran straight to the smog shop, otherwise it'd have to be redone. I take my reader with me each time to check right before it'll go on the machine. I know the code will pop back up but as long as it happens after the test, I don't care.