My wife took her 02 4Runner to a local mechanic and got the 02 sensor replaced in front of the first cat. After getting it replaced (or maybe before I dont know I didn't have an OBD II reader at the time) the truck started throwing out P0420. The mechanic said she needed to take the truck to Toyota to get the computer reflashed (which sounds to me like total bs) well skeptically I called Toyota and guess what the comp cant be reflashed only replaced to the tune of about $1200.

Anyway is it best practice to replace both the front and rear 02 sensors at the same time?

I guess the first thing I should check should be for a tight seal on the 02 sensor replaced. Whats the best way to check that? Soapy water with the engine running or completely remove and reattach?

~Shift

 

Btw the truck has 122k miles so letting the dealer deal with it is out of the question.

 

I think some have had problems with the brand of 02 sensors that they used to replace the factory ones. Search using that code and see what comes up.

 

That code is thrown due to an unexpected signal pattern from the rear O2 sensor, but it doesn't necessarily mean that the rear O2 sensor is bad. The catalyst in your catalytic converter(s) might be bad, or it could be the front O2 (A/F) sensor not properly controlling air to fuel ratio. First thing I'd check is what brand of O2 sensor did you get? I see you only have a couple of posts here, so I'm wondering if you've seen the multitude of threads here that say that Toyota's don't like aftermarket O2 sensors much. You want to make sure you got a Denso O2 sensor in there.

 

I did a bunch of looking around but I didn't see anything about them not liking aftermarket sensors. I'll check it out this weekend and post.
Thanks

 

I can't look up the threads right now, but there are several out there about non OEM o2 sensors causing codes.

Couple things here:
1st - Why did she go in too have the O2 sensor replaced in the first place?
2nd - Front Sensor (engine side of exhaust before cat(s)) is an AF sensor, NOT O2 sensor, so be careful of that.
3rd - p0420 code is a Catalyst (catalytic converter) not performing properly. Could be bad cat(s) or O2 sensor or AF sensor.

When you are checking I would also make sure that you check to make sure the tech didn't use a universal and try to wire it in and not use the proper connector, because then you won't be a happy camper.

You can get OEM direct replacements from Underdog Racing Development (search URD in google) for pretty cheap. AF sensor can be through them too, but I found that for AF sensor, TRDparts4u.com was about $40 cheaper.

Hope this helps a little.
Mark

 

"1st - Why did she go in too have the O2 sensor replaced in the first place?"

Her check engine light was on so she went to have the code pulled and left with a $500 bill grrr.

The mechanic said he replaced the 02 sensor in front of the cat is it possible to put an 02 sensor in the place of a AF sensor? Perhaps that could be my problem? I guess I'll find out tomorrow

 

the rear o2 sensor is bad. trust me.

it is old enough for sure, and when the front
is bad the rear one starts to die exponentially from the extra unburnt crud


replace it.

 

it is possible, but you'd have to dremel up the connector to make it work.

 

Did you see this thread on the same page?

https://www.yotatech.com/forums/f2/p...solved-169385/

Don't know if it would be any help.