2002 4Runner SR5 that's spent all its life in Los Angeles. Failed Smog. Ran drive sequence and the CE light finally came on.

P0420 code. every mechanic wants to replace the cat. I have gone ahead and ordered the Denso 234-9002 and 234-4261 and I will have a mechanic install them before jumpinbg into the cat. But they are telling me 90% of the time its the cat and not sensors (of course they would like to replace the cat as it's more $) so who knows?

After that ~$300 investment, I will be left with no choice but to find a cat that will hopefully let the car pass smog. One guy says replace the downstream cat first. Another says replace the upstream first. Who knows?? Either way, if that does the trick it's ~$1500 installed out of pocket for a Magnaflow/Walker/AP-Eastern direct-fit cat. To do what might be only half of what needs doing?

This has been a huge PITA and time waster. The worst part is going to several mechanics and getting different answers and different quotes from each of them. I'm thankful for the mostly consistent advice here guys.

Broken record time: I've been through the P0420s (a week before CA Star smog was due). Then 2 years later, I killed my battery a few weeks before smog and tried and tried and tried the official Toyota drive sequences and failed, failed, failed to set the Cat Readiness data. Both times, I had an imperceptible leak in my Magnaflow CAT-BACK bolt-together exhaust system. I emphasized cat-back because a leak beyond the second cat and sensor can throw a 420 and even weirder prevent Cat-ready data acquisition. I post this every year or so (to some raised eyebrows) in the hope that maybe just once it will save someone some money or grief. In my case a few cranks on the 14mm bolts solved both problems both times. I could not detect the leaks by feel, sound or smell. Believe me, I was expecting the worst. Yes, the smog guy said New Cat! So, what is the condition of your exhaust system? Stock? Rusty? Those sensors are pretty smart.....

They got it backwards. Many of us throw P0420 from time to time, and 90% of the time it's the O2 sensor - if it can't simply be cleared and returns quickly that is, which in itself seems pretty rare. Cats will throw P0420 if they're teetering on the efficiency level, which is totally understandable at the age they're approaching, but you have the right state of mind to tackle the far cheaper and far easier option first. Better to be out a few hundred bucks than $1500
I throw P0420 about once a year, maybe twice. I have cali emissions too. Every single time it's my rear O2 sensor. Wouldn't hurt to replace it if it's old, but if it's newer you could soak it in B12 Chemtool overnight. If the heating element is dirty inside it can drastically affect it's efficiency. B12 is O2 sensor safe so it won't harm anything. If you have access to an ultrasonic tank, throw it on in
If worse comes to worst you can throw on an O2 spacer, or even do so as a test - if the code STILL comes back then, I would start eyeing the cat

Been through exhaust leaks that have thrown the p0420 on two different occasions, and as of last month an aftermarket cat (410k on the clock). So basically the last 4 years I’ve dealt with this code every time smog was needed along with registration. Permatex Muffler & Tailpipe Sealer (rated for 2k degrees) and/or new o2 gaskets have been the solution. Brand new Denso O2 sensors haven’t solved any of my issues even though I threw some at the problem each time.

I’d personally check for exhaust leaks first. Black soot around the o2 bungs and exhaust flanges all the way from the rear o2 up to the headers can be a giveaway. To further check since the leaks can be subtle or our of sight, a shop vacuum blowing on reverse connected to the tailpipe (sealed up with tape) will pressurize the exhaust and you can spray those areas with soapy water. If there’s an exhaust leak, you’ll find it this way without using a smoke machine.

Recently, I used a buddy’s infrared temp gun to measure upstream and downstream cats at the inlets and outlets. If the inlet is substantially hotter that the outlet, it’s a sign the cat isn’t working right. That led me to the rear cat being bad on my 2000 and $580 installed for a Catco aftermarket weld-in cat from a local exhaust shop got me to get my catalyst monitor set within 2 days. It was the last monitor I needed to pass smog. I’m glad I didn’t have to do both cats at the same time, and this pinpointed the bad one pretty quickly. A $20 obd2 code reader is a good investment.

Good luck!

Mine sets a P0420 if the traffic conditions are JUST right - but still blows great numbers out the tail pipe and has for a few years now. I just have to carefully set the monitors if it is near smog check time.

I think at this point I should probably get new AF/O2 sensors, just because... and check all those exhaust joints since it has been removed in the semi-recent past for the 4WD swap.

-Charlie

Sometimes I wish CA still did a sniffer test on all vehicles but 2000s and on are snifferless & sensor data only.

A muffler or exhaust pipe hole would cause code 420 (bad cat)? I would think that the less fumes (exhaust) would be a good thing out the the tail pipe?
I'm in CA and have been with this problem every 2 years and have fenageled my way through it. A pain. It's time again. I'm hoping to outsmart the sensors again.

Here's another smog test "broken record": Make sure to drive it until it's thoroughly warmed up right before the smog test. Like 40 minutes on the highway warm.