My 2000 4Runner (4x4 california emission, 76K) recently had DTC PO420 poped up. I replaced front Air/Fuel sensor and cleared the code since then. However the same code appeared again and again. I figured the previous fix did not work. The rear O2 might be the real cause (I am so scared that I do not dare to think of the cat coverter!!!) Before I cough up another hundred bucks to buy a rear O2 sensor, can anybody tell me how can I know if a O2 sensor is bad or good? (The FSM states the method to check the resistance of the sensor, e.g. 0.8 -1.4 ohmm. However I measured it on a new front O2 sensor I got from sparkplugs.com, the value is 0?????????) Anybody fixed PO420 before?

Haven't had to deal with a cat failure yet, and since you have the FSM, not much I can add.. The only thing I would point out is the statement in the FSM on page DI-247..



I would also interpret the above statement to also mean that if the back sensor is slowing down in its response, it would in effect give the catalyst the benefit of the doubt, or maybe not even flag a bad catalytic converter..

When I replaced my front O2 sensor I did ask my trusted service manager (no pun intended) if the back O2 sensor should be replaced.. Got one of those.... "NO" shake of the head saying don't bother, but you never heard me say it <img src="/forums/images/graemlins/ignore.gif" alt="" /> ...

So I don't ever plan to replace my back one.. unless it starts throwing it's own error codes...

So my guess would be the catalytic converter.., but would certainly defer to anyone else who's had this problem...

Others with experience???

And on the ohm meter measurement, were you using the lowest ohm scale setting on your meter (or an auto ranging DVM) when you measured it? This measurement is on the heater circuit, which the ECU should be able to determine very accurately if it's good or bad based on measured current flow..

On your back O2 sensor the heater circuit will have a higher resistance per the FSM on page SF-55.. Acceptable range: 11 - 16 ohms.. I just measured my old one (from the front, which non California spec vehicles use the same sensor front and back) and it was 13.94 ohms, and as I read the FSM, is the same on the rear of the California spec vehicles..

This measurement tells you nothing about the responsiveness of the sensor (as discussed in this article), but as discussed above, I believe a slower responding rear sensor would in essence be giving your catalytic converter the benefit of the doubt..

I just had my check engine light come on and I got a handheld diag device and it said p0171 system running too lean so I called Toyota of Seattle and told them I wanted to have them look at it since I bought a extended waranty and they did and determinded that it was the 02 sensor and they replaced it easy the waranty took care of all but the $50 deductable.

Thanks for your detail explanation, Dan. Yes I did use an auto ranging DVM I just bought as new from Sears. The only thing I can guess is that my DVM may not be able to meausure that tiny value. Otherwise I can hardly believe that a brand new front O2 sensor is bad.

Anybody fixed PO420 before??? I guess I'll end up with replacing rear O2 sensor if I have no other choice %&*!@#$!