Hi everybody.

Need advice, so I’ve had my new used 2007 Toyota Prius for 3 weeks and in week one it had p0420 code, the seller had already put a brand new cat on the car just b4 I thought it. I’ve since replaced the rear o2 sensor and the pre cat a/f sensor, I also replaced the spark plugs which were horrendously crusted up. Today my error code Came back. The only other thing I can think is that the coil packs have gone although the engine does not run that lumpy. Any advice, ideas, I’m stumped now.

Cheers.

Thank you for that detailed reply fredrick

P0420 is the only code and do far I’ve replaced both O2 sensors, spark plugs and maf sensor. I’ve also checked the coil packs whilst engine running.

This is the error

[imghttps://imgur.com/a/PEjYDimg]

Quote from F-150Torqued on March 21, 2018
If those other things were the source of your Code – it is my firm belief you would have a different code. Trust what the PCM is telling you. It is the closest to the situation,

Having said the prior owner changed the CAT, I would be looking very carefully into the model number of that replaced cat and its compatibility with your vehicle.

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54371019

OK thank you very much for your reply

I’m sorry @Luke. That’s a problem. Auto manufacturers & politicians have BOXED us in completely. You can’t even cut the trace to the check engine light. The circuit is monitored by the PCM and that will produce a different DTC. (LOL). And here in the US you wouldn’t be able to get an inspection sticker to drive the car.

I do not know if the previous owner replaced the CAT without promptly fixing whatever damaged the original, or if the aftermarket replacement has compatibility issues. But I trust the PCMs report on what is going on (Switching ratio of downstream O2 sensor -to- upstream 02 sensor is exceeding factory spec). Only thing I can think of that _MIGHT_ mask your problem enough to make a difference. There are adapters that screw into the O2 sensor threads, then the O2 sensor screws into the adapter to hold the O2 sensor ‘slightly’ out of the exhaust stream. (I think they are called ‘Spark Plug Adapters’). The idea being to slow down the O2’s ability to sense changes in oxygen content. You might try one of those.

Or check on a warranty or factory defect claim on the replacement cat.

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54371019

Quote from Luke on March 24, 2018
So the cat is the problem but it is definitely a Toyota Prius catalytic converter so why is it still throwing the code? It’s not a genuine Toyota cat but an aftermarket version. I can’t afford 1600 pounds for the real thing.

As people have said checking for exhaust leaks is a good idea.

You can get an extension to go on the 02 sensor like a threaded socket that has a small amount of catalyst in the tip that threads into the socket in the exhaust. Where I live we call them hedgehogs I have no idea why. Look on ebay. This will clear the P0420

Serious misfiring can kill catalytic converters, if they had misfire problem that killed the original cat, that they didn’t rectify prior to changing the cat, it could have killed the new cat.