Upper O2 sensor replaced but did not fix the problem

[Hard_Yaris]

OK... HELP please..

I am still getting the po420 code after replacing the sensor closest to the motor. Right away I noticed the increase in performance... then i tripped the code again...

Do I have to replace the second sensor as well...? My car is at 160,000Km. original exhaust...

What should I do to fix the code?

[CTScott]

It is very possible that the rear is bad as well. P0420 can be set by either or both being bad or by an exhaust leak between the block and the cat.

[Hard_Yaris]

thank you CTScott... ill check for leaks soon.

[Hard_Yaris]

what's the best way to check for leaks in the exhaust?

[CTScott]

Originally Posted by Hard_Yaris

what's the best way to check for leaks in the exhaust?

I have heard that seafoam is a great leak finder, as it makes the car smoke like crazy while it is doing its cleaning work, so you can easily see leak points.

[YarisSedan]

P0420 9 times out of ten is a bad catalytic converter. Which can be cause by a bad o2 or worn out plug as some of the common causes. I would take your car to a muffler shop and see what they can do to help you. You can probably find a aftermarket one for cheap since you dont need a California approved cat. Typically about 200 installed.

[CTScott]

Originally Posted by YarisSedan

P0420 9 times out of ten is a bad catalytic converter. Which can be cause by a bad o2 or worn out plug as some of the common causes. I would take your car to a muffler shop and see what they can do to help you. You can probably find a aftermarket one for cheap since you dont need a California approved cat. Typically about 200 installed.

160,000 km is pretty young for a bad cat on a Toyota.

[YarisSedan]

It does seem early but if it were a 02 sensor typically it will be accompanied by additional codes. The code p0420 is triggered when the 02 level measure before the cat and afterwards are similar waveform patterns indicating the catalytic converter is not doing its job anymore. This obviously can be from a defective sensor but usually when the sensor starts to get lazy and slow down on its signal it will trigger a fault code 02 sensor slow or no response, that or a o2 sensor heater circuit fault. If the 02 sensor fault is set it wont run the system monitor for the catalytic converter i belive so after you fix the o2 then it might set a p0420 but usually not before. If we assume the front 02 sensor is good since its been replaced then that owuld mean if the rear o2 sensor isnt switching it wont set a p0420 because if the rear 02 was bad it wouldn't be switching or sending a signal at all it would have to mimic the front 02 sensor but if it was bad that would not be possible. Or at-least very unlikely. Which is why i would guess it is most likely the catalytic converter. Granted at that low miles that seems unusual there probably is something that caused the cat to go bad. Could have been a lazy front 02 that was working good enough to not trigger a 02 code but causing the vehicle to run richer than usual and over a long period of time led to premature catalyst failure. Maybe the vehicle had a missfire at one time or another such as a bad ignition coil causing cat damage, maybe the driver has been using premium gas instead of 87 octane causing incomplete combustion consistently would hard to know without service history.

[Hard_Yaris]

This is all very helpful thanks!
I do run premium gas when the car is loaded up and driving in the mountains, probably not good but it helps pull the car up the hills better. so i don't run it all the time.

And i do live in Edmonton where the winters are long and hard on cars.

so your saying I don't need to get an expensive cat? Most of the ones I got quoted were $500-700.
Which one would you recommend?

[esse10]

You should check anything that would effect your emissions effeciancy and with that many miles I would check & clean the throttle body valve& MAF sensor. If it has never been cleaned before rest asure it's got lot's of gunk on it. Then reset your cpu by disconnecting the battery for a while, then reconnect because the MAF sensor will try to re-adjust air flow again and the idle might be off after the first few cold starts, then it'll smooth out. Good luck man