2004 prius p0420 bank 1 code

How many miles on this vehicle?

The potential cause for failure is important, and having a 'mini-vci' cable and techstream (dealer technician software) may tell you.

Not sure which part of AZ you're in, your profile makes no mention. Best to find or procure a toyota hybrid compatible OBD2 scanner and post the OBD2 codes (DTCs) here for further guidance.

There are better methods for soaking a catalytic converter, though please reference any sources if you know otherwise. @m.wynn has posted a protocol (researched and peer published elsewhere) where he had limited success : Catalytic Converter Issue..In California of course.... | PriusChat

Neither the cat or A/F sensor will last a lifetime, and since you're in AZ, best to just replace both (250 usd) and be done with it.

Not sure if the sensors would benefit from soaking, but highly doubtful. Please post anything that you've found that says otherwise.

 

Just the upstream A/F sensor and the catalytic converter. My last Denso A/F sensor was purchased from CarId.com, they'll price match anyone, even if the virtual hand holding takes time. Since you aren't in CA/NY/ME you can use an aftermarket cat, many of the $120 fleabay offerings from Canada will work just fine.

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For your OBD2 device and this application, 'bank1' will mean cylinders 1-4. There are two (2) related sensors, and you should focus on the upstream A/F sensor, unless the downstream sensor has coded. Have you posted all of the DTC's that you've retrieved?

 

You need to immediately remove your OEM cat and install an aftermarket before it gets cut out of the car by a thief. Install an aftermarket and sell or keep the OEM in case you need it for inspection. The theft is not an if, it's a when.

We just had a large article in our state newspaper about how cat thefts are ravaging the Columbia area in South Carolina, with the Prius especially hard hit, so the disease has even spread here.

 

The aftermarket models have 2 different designs. One is a full replacement where you just remove 2 bolts at the exhaust manifold and 2 bolts at the pipe joint near the rear wheels. The other is just the first 3 or 4 feet where the cats are. It would require cutting the exhaust pipe about 2.5 inches ahead of the resonator, and then remove the 2 bolts at the exhaust manifold. The new cat pipe slips over the 2.5" section of pipe and attaches to the manifold using the bolts previously removed, and is sealed with a U shaped pipe clamp.

 

2 just like the original.
Search on ebay using "Prius catalytic converter".
Dozens of options will show up

 

Toyota and Subaru are notorious for logging P0420 faults even when the engine and CAT are operating perfectly. They have VERY tight performance tolerances that can easily be exceeded by maintenance lapses (tune up, oil consumption/contamination, etc).

I have been down this road with both brands more than a few times each and the #1 first thing is to ensure there is no maintenance due (like plugs). Next most likely cause is a worn out oxygen sensor, they are NOT life-of-vehicle, oftentimes requiring two replacements over 250k that these cars often get. DO NOT substitute "might fit" aftermarket parts, while tempting, doing so can lead one to the potentially FALSE conclusion that more repairs are needed when the CEL comes back; all-the-while due to substandard crap new parts being used.

With O2 sensors, and these cars have two, get them both; if it does not say "BOSCH" or "DENSO", and/or it is not a direct plug-in using the factory connectors, forget it (aka: RUN to your 'Yota dealer and get them there).

Both O2 sensors can be tested to determine their functionality and there are a great number of videos as to how, BUT they are like sparkplugs, at some point they NEED to be replaced so if not already, just do it. It will likely improve your FE.

A dirty MAF sensor can also cause p0420, because the engine is getting more air than the computer "sees", combined with even a small error in the O2 sensor reading, and wham-o.

Exhaust leaks ahead of the O2 sensors can also cause p0420, not super common on Toyota, but something to be aware of because you typically do not need parts to fix this.

And yes, aftermarket CATs can work, but thus far I have seen no evidence they live more than a few years. Remember, Toyota and Subaru have TIGHT fault limits, so just because an aftermarket CAT meets the performance when brand new, does not mean it will next month.

 

If a Toyota does have an exhaust leak upstream of the O2 sensors, then I have usually seen it occur at the spring-bolted flex joint because the gasket is worn. The only replacement that I have found to properly fit and seal is the one from the dealer.

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I have a 05 Toyota Prius with only 90,000 miles and P 4020 code came up. I was going to pour CATAClean but the engine light went away before I could add it to the gas tank. I still pour the additive into my gas tank when my gas tank went to 1/4 just to be on the safe side. However, a week later the code came back up. It is hard to believe it could be the cat as the vehicle only has 90,000 miles, I am going to clean the MAF sensor and the oxygen sensor hopefully that would fix it. Do you think pouring lacquer into the gas tank would work as my last resort?

 

As strange as it is, I recently helped someone who had a 2007 with only 53kish miles. Also a P0420 and when I removed the cat, it had some really weird markings/patterns on the visible end of the converter material. It may have been severely overheated. They had some unusual engine 'running' problems that were likely created by a repair shop, which eventually led the shop to replace the fuel tank. I ended up going through the car and re-installing the original fuel tank. Unfortunately, it did need a replacement catalytic converter.

If you have a good air compressor, you can remove the cat assembly and blow high volume air through it from the O2 sensor port and each end. You'll probably be amazed at how much carbon dust comes out. Don't have the 'blowing' end pointed at anything important. It will also let you take a good peek at the inlet end so you can see if there's any unusual buildup that may be clogging it.

 

Putting who-knows-what in the tank might not be very good for the fuel pump or injectors.

I go through a diagnostic checklist for P0420.

1) No exhaust leaks (even small ones)ahead of or near the downstream O2 sensors. No physical damage to the exhaust system
2)No misfires (can kill a cat fast)
3)Air Fuel and O2 sensors can respond with "full range" readings.
4)Fuel Trim data is "normal" (0 +/- 10% under all driving conditions)
5)Oil use - consumption (burning) will eventually kill a new cat

If all that passes, then I replace the catalytic converter.

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