2000 Toyota 4 runner, changed both o2 sensors light comes back on going up slight incline P0420 what's next gas cap? Or cat?

code p0420 refers to the cat converter system-bank 1--efficiency below threshold===causes --wiring--cat converter -- HO2S 2. I would replace the heated oxygen sensor number 2. There is no real life on cat converters as they are extremely fragile and run at extreme temperatures which means that if you hit a icy puddle of water / snow and the cat is hot they are prone to cracking and disintergrating internally and blocking up the exhaust pipe.. In your case I would be looking at the sensor. Use the correct tool so that the wires are no damaged.

Has to be diagnosed for actual problem, because it could be a vacuum leak, cat, or O2.

You have a fail exhaust converter on bank # 1

Google the PO420 code & read up

You need a catalytic conveyer. Before you buy one contact your local dealer. These are covered under the federal emissions warranty for at least 8 years or 80,000 miles. The upstream O2 sensor has nothing to do with monitoring the cat. The downstream O2 sensor is the sensor that monitors the cat. The MAF sensor would set lean codes if it where bad or dirty and you would have noticed a lack of power under acceleration if it where really bad. The gas cap has nothing to tpdo with tailpipe emissions so I'm not sure why that was replaced. I guess what I am getting at is you could have put all the money you a lady spent towards a cat and the problem would have been solved. Let me know if you have any questions and I'll do my best to answer them. Thank you for using FixYa.com!

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Hello: Code 420 normally indicates a catalytic converter is operating below 60% efficiency. You need to take your car to another shop and request a COMPLETE diagnosis not a dart board guess.

The after market cat is the likely problem. Catalytic converters are not all the same and over time our shop has learned which aftermarket ones will work and which ones will not.

The computer is looking at the downstream O2 sensor (the one after the cat) to determine cat efficiency. If the downstream O2 sensor is switching simular to the upstream O2 sensor (the one before the cat) than the computer interprets this as a bad cat. The rear O2 sensor should hold a steady voltage at idle once the car is warmed up.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

PO128 = It is probably a bad ECT (engine coolant temp) sensor. PO420= The problem could be a bad cat, o2 sensor (Pre- and/or post cat)or o2 sensor wires, or an air fuel mixture problem that has caused the cat to fail.PO430= Catalyst System Efficiency Below Threshold (Bank 2) Your CAT converter is not cleaning properly. Hope this helps, James Booth

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Code P0420 is a code for Catalyst efficiency. The computer is seeing the post Catalytic converter o2 sensor having too much activity. The Cat. should be cleaning the exaust of Hc, co, and Nox. The post cat o2 will read the o2 content in the exhaust and if it moves up and down too much or to high and low, the computer interperates that as the cat not ridding the exhaust of it's pollutants. This indicates that the converter is unable to preform properly. The problem could be a bad cat, o2 sensor (Pre- and/or post cat)or o2 sensor wires, or an air fuel mixture problem that has caused the cat to fail. Many other engine operating system failures can cause the converter to not work properly.

well thank you very much you have me blushing : ) i went to school for automotive and after seeing people be taken advantage of till it almost made me sick i chose to take my own business path, I might not be the best mech in the biz, but i would like to think im the most honest, i have trouble charging people ive never met before lol and usually give them 1/2 the price a normal shop would with out marking up my parts 50% like most shops also, once again thank you.to explain what is happening, the heated o2 sensor between the engine takes a waveform (voltage reading to determin the air/fuel mixture) then the mixture travels through the cat and is cleaned and comes out to the 2nd post o2 sensor, the computer checks these to make sure they are different and that the cat is doing its job by changine (cleaning) the exhaust flow. the reason you are having this code thrown is because both sensors are reading the same which means that the converter isnt doing its job. now this means that the cat is bad, the heated #2 o2 sensor is defective, or the #2 o2 sensor circut is open/shorted.Tthe answer to your problem to the best of my knowledge would first make sure that is the only code that your car is throwing (to make sure another problem isnt causing this one) if p0420 is the only code showing you will want to check through the entire exhaust from the engine exhaust housing back to make sure you have no exhaust leaks. you may have a p0130 which is the heated o2 in the front or a p0136 which is a problem with the other o2 post cat. these codes should show, if they dont i would still go over the wiring of the sensors from them to the ecu make sure they are not shorted, if you go to a shop with a scan tool they should be able to check the o2's with it and make sure your fuel trims are where they should be also. if only the p0420 i would guess there is a leak somewhere, all the heat cold heat cold back and forth can loosen bolts and strech metal joints in the exhaust system. if that doesnt help please let me know and i will research it more much harder, but this is what the manual for your car says and what i believe also, please let me know and i'll get back to you right away, also make sure the cat didnt just randomly go bad, but dont let them replace stuff say you want it tested and make sure they have the right tools to do it. take care and thanks very much again. any other problems with anything else send them my way first, take care have a good night

Erik
EZ Automotive/U.S. Army