Persistent P0420 even after o2 sensor replacement

Was it replaced with a Toyota cat?

 

Yes that's my question too so you're in California It got stolen in California and it got rectified in California? So I must assume in that heavily regulated state that car had to physically go to a Toyota dealer a dealer part was ordered which was just under two grand or close and then it was installed at that said Toyota dealer and then you left the dealer and how long till the light came on with this code? And then since then they've just shrugged it off and sent you away? Or what has happened? Or do you have the magnaflow cat that was approved? Lot of things that can vary here but generally in your state there are no variances allowed apparently of course I know better than that but that's the law

 

I would want the test results that show how the 12V battery is bad before replacing it.

Then I would want a "layman's" explanation as to how the 12V battery is causing the ECM to set a P0420. A weak battery can cause grief if the voltage goes too low during "startup", but I have never seen it result in a false P0420.

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Every shop always wants to replace a battery on a Prius whichever one they're touting will fix the car it's always the 12 volt or the HV battery that seems to be the magic fix for the whole car it's actually quite comical.

 

Welcome to PriusChat!!

How many miles do you have now?

FYI : you're moderated you've posted 5 times.

 

Well if it is a P0037, that code is for the heater circuit of the "downstream" O2 sensor. Assuming that sensor was just replaced by the dealer, you need to find someone who can actually do some testing to figure out what is going on. Could be a wiring fault, or maybe a damaged driver in the ECM.

Unfortunately, many shops have techs that only know how to fire the parts cannon at a problem.

Good luck.

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Same here with two different cats one factory one not . No difference

 

Ehh??

In post #1 you said the dealer replaced the downstream O2 sensor. Now you replaced both the A/F and the O2, then "put back the old ones"??

So are any codes resetting or not? Was there a P0420 or a P0037?

I really don't understand what has been going on with your car or what you have done. Much of what you posted does not make any sense.

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You didn't explain how did you replace the cat after it was stolen.
Assuming you didn't put a new OEM original replacement, and put either a used OEM or non-oem used, then it can cause to show this code. I had it too when my mechanic tried to scam me and put a used OEM instead of the new one (it's illegal to sell a used catalytic in California). See my thread details about it:

Could I sue my mechanic for potentially installing an aftermarket catalytic converter? | PriusChat

 

Overall, I find it difficult to believe a Toyota dealer could not resolve this rather simple problem if they used factory parts, or if they did the work at all. Sometimes a body shop does the cat replacement and botch up the wiring.

If an aftermarket Bosch or similar o2 sensor was used by the "dealer" the wire colors don't match and the tech has to use a table to get them right. Second the long factory o2 cable would have been cut in most cases and the factory connector to the wiring harness is inside the cabin. It could be damaged. Or the cable itself could be damaged upstream of a splice made in the original repair.

 

What I find funny about all this is on all my other Toyotas and some other branded vehicles also is when O2 sensor wires have been cut ripped out by road debris what have you I've never had any trouble joining wires together to a plug even the braided shielded wire daily and the heat wires never a problem I can name off cars left and right corolla's up to 02 all that never an issue The only two cars that I've had to check engine light on for years is this 09 Prius and a 02 Corolla with the AC Delco 1Z engine system the ACDelco computer got an update about 9 years later for firmware that supposedly fix the problem I never got it because they wanted $200 for it. The '09 Prius that I drive now has a factory converter on it someone tried to cut the pipe they literally did not get their saws all through the rear most section of the cat they just got their saw up there and got scared and left didn't cut anything didn't move any wires nothing. The light came on the next day PO420 I took it to my friend's muffler shop. Were they lifted the car up in the air as usual I saw the mark where the sawzall got laid against the pipe in the trigger pulled No hole. My buddy put some welding snot around it anyway just to be sure there was no leaks car running on the rack no leaks no noise O2 sensor connected everything looks good like it always has. The code still remains when I reset the code and clear it sometimes it takes a few days while at least a good few hours for the code to come back it's not instantaneous like something is instantly wrong so it must pass a few of the tests that the car runs on the converter while starting.? Or something along those lines I'm going to replace the downstream O2 sensor on this Prius one of these days but I don't expect the serious change buying a $3,000 catalytic converter to put a light out that's not even required in my state is out of the question I could buy another whole car another 09 Prius actually that doesn't have that problem. As long as it does not affect my gas mileage MPG where I am in my state has no emissions so the check engine light is irrelevant. If it was putting me in the limp home mode and I was getting 39 mi to the gallon then I would be on it.