Hey guys, my check engine light came on last night. My mechanic checked and its throwing the code P0420. I know this could mean catalytic converter or O2 sensor. Is there a way to tell which one it is though? Also how much should I be looking at if its the cat or the O2 sensor?

Attached are pictures of the O2 sensors. Can you guys tell by looking at them if they are good or bad? The mechanic told me if its the O2 sensor that went bad then the tip should be black and as you can see in the pic the tips are not black. what do you guys think?

google it, amazon, ebay
what sensor u need?bank #? sensor #?
for cat? u looking @ 200+ direct bolt on,
i went custom tho for my cat

I heard to stay away from after market cat and go with oem, did you have any problems with your custom built cat? What is the reason with a custom built cat?

Before you buy anything get a digital multimeter / tester and find a wiring diagram for your car. If you can't find the wiring for the o2 sensor let me know. You need to get the leads into the connector for the o2 sensor (there also could be a A/F ratio sensor which is in front of the cat, which operates similarly) on the proper wires and check the voltage output of the sensor. I believe these are heated sensors in your car, so there are 4 wires in the connector. two are for the powering the sensor and the other two are for voltage feedback to the computer. The latter is what you need to check. If the sensor is working properly you should see the voltage constantly moving and changing. These sensors usually malfunction one of two ways: either it will be "open" and no voltage will be displayed, or the voltage will be frozen. In my experience they fail "frozen" and display a constant voltage which the computer does not like.

Also in my experience cat. converters rarely fail on Toyota products unless the engine has been allowed to blow an unusual amount of burned oil down the exhaust and coat the platinum grid in the cat. If you have any electrical know-how this should be pretty easy. I would send you a pic of the diagram right now but a little busy, sorry. Let us know how you make out.

One last note, if you end up needing an o2 sensor, don't buy aftermarket BOSH sensors. Check out this site:

http://www.toyotapartsales.com/partl...?siteid=214074

I have used it for years, and they have genuine toyota parts at prices you can actually afford. It's great. I've bought several "Denso" OEM sensors from them for less than the BOSH crap aftermarket sensors. They have been great to me and even called me about an order due to my 4Runner being a california emissions, which I didn't know, and they resent the proper sensor and paid for the shipping to return the wrong one which I ordered, my mistake. I think it is the parts dept. of Toyota of dallas. They will ship anything except motors, tranny's and body panels.

Hope all this helps.

Thank you for taking the time to reply. I don't have access to a digital multimeter tester so I wont be able to do the test. But the day I went to go see the mechanic he reset the light and I believe he said he cleaned the intake and few other things and the light hasn't come back on and its been a week now.

I want to go ahead and change the o2 sensors though as I have been getting bad gas millage the last couple of months. I believe there are 3 O2 sensors, am I correct? Can you please provide me with part #'s for the Denso sensors please? Also what are the air/fuel ratio sensors? Do i need to change them as well? if so can you please provide me with part #'s as well? Thank you.

O2 sensors and air/fuel sensors are the same thing, interchangable verbage. And there are three of them, 2 upstream (before the cat convert.) and 1 down stream (after the cat convert). I dont know the part number, but just be aware there are different part numbers depending on where it was sold. My 98 ES300 is a California Lexus and the smog requirements are the strictest, so my part number for my sensors are going to be different for a non Calif. ES300. That being said the price is different too, Calif. sensors are more expensive.

How can I tell if my es is a california lexus. Will it say on the door?

In my experience at the dealer we would replace catalytic and 02 Sensor of course diagnose to make sure. DVOM is the right way to go get a schematic and do the tests.

here the deal . . . are code read engine off . . . or engine on ? it makes a big difference . . . because egr can initiate 420 . . . but not with engine off . . .is the vehicle running extremely smooth ?

Dont know about this one Sir, sorry, hopefully someone on here might have that answer.

There is a sticker under the hood and it says it on there.

The engine light never came back on but I went ahead and changed all three o2 sensors and car runs great now.