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Help - Error Code P0420

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If the dealer really did replace the whole system, then that wouldn't (shouldn't) be the case, right?

If the dealer really did replace the whole system, then that wouldn't (shouldn't) be the case, right?
If the dealer replaced them and they are below threshold again, then something else is wrong, causing them to fail.

Did the dealer replace both pre and post cat O2 sensors? Did the dealer replace both cats with factory or aftermarket cats? Did the dealer do any diagnostic work to check O2 functioning, O2 wiring, and/or a fueling system issue?

Some back door hacks you can do is the spark plug non-fouler trick or just turn the code off. But I would do more diagnostic work if indeed you have brand new O2 sensors and cats installed because then you probably have a fuel system issue, mechanical issue, exhaust leak, wiring issue, etc.

O2 sensors rarely cause P0420/P0420 DTCS. Those codes are generated AFTER the O2 sensors have already been tested and indicate that the catalytic converters are no longer converting enough to meet emissions standards. Now, faulty upstream o2 sensors will cause the converters to fail if they cause the engine to run rich as will a host of other things.

Pull your paperwork, if you can, and see (post) exactly what the dealer replaced and what parts they used. I.E., OEM vs aftermarket.


I bought the car a couple of years ago in Atlanta. Later, I was doing some maintenance to the underside and discovered that someone had put in one of the upstream O2 sensor extentions to fool the ECM into not seeing a P0420. Probably the dealer to pass emissions. I should have checked before I bought it, dangit! Shortly after that I changed the bank 2 catalytic converter. $100 and couplde hours of my time. The P0420 has not came back. P0430 started intermittenly about 6-8 months ago. Off and on.

Forscan & Torque will not let you monitor the upstream O2 sensors of Bank 1 & Bank 2. My fuel trims look very good. Not anywhere near the plus/minus 10%.

The fix you posted didn't work. I tried it, but not much of a snake oil repair person. I like it done right.

Hello I though I could finally rest but here I comes again :(
my vehicle it giving me more problem in the last month than it have gave me in the last two years, however I love this vehicle so I want it to have it in like new condition.
I was driving my vehicle after taking out from dealer and suddently check engine warning lights turn on, since I do not have time to return to dealer I drove home, I plugged my personal OBDII bluetooth scan and gave me the following error :
"P04020 Catalyst System Efficiency Below Threshold"
I have done a resarch in the forum and learn this code might be triggered by a multiple and different causes. But I also noticed that all thread in the forum are for previous explorer generation, I cannot find a thread for 2011 or newer models..
I would like to know if Cause of the problem remain the same in my model as on previous and older explorer models. Before take the vehicle back to the dealer, I would like to know if there is something I can do my self at home to repair or fix this problem.
Botton line, vehicle has 109k miles, I erased the code from the vehicle to turn off engine light
thank you in advance for your assistance

I was strugling with that problem more than one year in my 2019 Sport but all solutions failed. I got P0420/430 on both catalyst units and extended test from Forscan detected to much oxygen on both banks. So I replaced cat units with dedicated performance parts and disable second lambdas checking ( this was done by external company ). Have no idea what was the cause. No airleaks, no fuel system problems, fuel/air ratio ok. Really nothing to catch on :( I wil come back to this in future but I was tired with finghting that s..t and spending more and more money to find solutiuon.