PO420 code = Bad cat or O2 sensor?
 

Well I got my code for the check engine light that popped up during my last wheeling trip in the mtns, "PO420 catalyst efficiency below threshold bank 1"

After searching here, it seems there are two solutions:replace O2 sensor(which one?) or replace catalytic converter(I think I have 2 in CA?)

Does anyone know which one is usually the culprit with this code? Or how to check if the O2's are working properly? Are there any negative effects to not getting this repaired right away?

Not sure if this matters, but my exhaust pipes are broken off on the exit side of the muffler. They got damaged last time I went wheeling up there and have been banging around for awhile, after this trip they got worse.

Last but not least, any leads on cheap online sources for the sensor or cat would really help out!

o2 Sensor could be that or a dirty MAF sensor

Why does everyone keep recommending URD? That is not the cheapest O2 sensor around, and I've barely looked. I think someone even found a universal one that you use your existing plug with for less than $60.

Maybe I'll get some extra ponies out of the URD one?

Searching around here will show that the universal ones have been more trouble than the $29.95 you'd save from buying one.

OEM is the tried and true recommendation for O2 sensors. OEM is Denso, URD has good prices on Denso.

Use anything else other than Denso at the risk of having to chuck it in the trash and buy Denso the next day.


and... A P0420 is "bank 1", "bank 1" is the FRONT O2 sensor.

Any chance it's both the O2 and the cat? Or does the fact that it's upstream of the cat mean it's just the sensor?

There's always a "chance". :) In the end, the cost of the cat is about the same as the O2 sensor when you factor in the additional cost of having a cat installed.

That said, the O2 sensor is "owner installable" in about 10 minutes, versus having to head to an exhaust shop to get the cat done. (I'm figuring that you don't have a welder...)

A couple of questions can sway the guess on which is more likely to be bad...

• How many miles are on the present cat? (has it ever been changed?)
• How many miles are on the present O2 sensor? (has it ever been changed?)

If the answer is the same for both (so, same mileage and they're both original) then I'd bet on the O2 sensor.

The other thing to bring up is that you're on a '99, and you're in CA. That front sensor is probably a wideband A/FR not "just" an O2 sensor. That'll cost you an additional $100. The price is still balanced between the sensor and the cat, since your cat just got more expensive as well.

*cough*.....actually bank 1 is the side (the 3.4L only has 1 bank of O2 sensors, not related to the number of cylinders), so sensor 1 would be the front sensor......*cough*

Mark has very good advice on the sensor, use a Denso or you might be replacing it again to fix the problem.

Also a hole in the exhaust before the cats can definately cause it to throw a code like P0420, get that fixed first and foremost before doing any other parts replacement.

crap... you're right. I saw "bank" and read "sensor".

Thanks Jamie!

Rob--

Maybe this will help with your diagnosis if the light comes back on...

http://www.4x4wire.com/toyota/4Runner/tech/O2_Sensor/

Well so far it hasn't come back on. I reset it on Wed. evening and I've made over 6 trips since then.

Could it have been the elevation and the broken exhaust pipe letting in more air?

Lucky dog. My code came on after eating dust during the same trip. Cleared it today but a few hours later it came back. I'm going to do the 02 sensor first as the cat for California spec vehicles is pricey.

^ did you try cleaning your MAF