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My 2006 Toyota Prius with 280k miles is throwing the P0420 code, which means that it (probably) needs a new catalytic converter. I live in California, which makes life much more difficult to replace a cat. The direct fit CARB-compliant catalytic converter cost about $1500 in the Toyota dealers (from I could search, still didn't asked to one directly), which is way too expensive compared to welding a universal catalytic converter (about $300). Will it still pass smog with a universal catalytic converted welded in?

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    This is really bordering on being off-topic. I think the answer is, the cat has to be legal in CA. As long as it will pass CARB, it should be just fine. If it won't, you better get a stock one. The reason it's close to being off topic, is because it's about price-shopping assistance. – Pᴀᴜʟsᴛᴇʀ2 Dec 15 '15 at 0:49
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    This is not a price-shopping assistance, this is: "Will doing this fix my car?". Otherwise the standard answer for all questions here would be: "Take it to the dealer and they will fix it." – Gabriel Diego Dec 15 '15 at 1:05
  • Have you even tested the cat to ensure it's bad? Along with what @racefever said about the O2's, you can test the cat to see how it's functioning with a laser thermometer. Get the temps at the front inlet and the outlet of the cat, right at the joint of where the pipe meets the cat body. With the cat fully warmed up, it should be about 200°F hotter on the outlet side than the inlet. If they are about the same, or the inlet is actually hotter, then the cat is bad. This may be hard to do with a Prius, considering it will run off the battery when it can. Other option? Move out of California. ;-) – Pᴀᴜʟsᴛᴇʀ2 Dec 15 '15 at 1:26
  • Thanks for the information, I will check all those details once I go back to my mechanic shop (he could not do the job today, just checked the codes). I found some more information about which kind of cat can I install legally on this link here. If that information is true, this answers my question. Of course that I will just replace the cat if everything else fails. – Gabriel Diego Dec 15 '15 at 1:31
  • You can also do your own shopping. Most every exhaust manufacturer will sell cats. The larger companies will sell ones which are CARB certified. You'll have to do your homework, but by the sounds of it, you're not afraid of a little leg work. At the cost of a new dealer installed one, I can agree with what you're getting at. – Pᴀᴜʟsᴛᴇʀ2 Dec 15 '15 at 1:36
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In short: no.

To pass smog in California (and other CARB adopting states) the catalytic converter must be the one from the dealership or one that is in the Aftermarket Catalytic Converter Database. The catalytic converter must be one of those listed for the Toyota Prius, but unfortunately there is none for the Prius.

For now the only possibility is to try other solutions like using a catalytic converter cleaning additive, washing the catalytic converter or make a tune up in the hope that one of those will help.

Edit

For those curious with the end of my ordeal, after months of poke and hope, I changed spark plugs, poured Cataclean and more recently poured Seafoam and it worked! At least long enough to me go to smog and have the car certified. After 3 miles it came back again :-(

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Try a junkyard/recycling center before buying the part new. You can find one in good condition for much less. If you want a new part, ordering it online is usually cheaper. This is not something that the dealership needs to do. Have your mechanic remove the old and install the new. Then delete/reset the codes.

Has your mechanic tested the O2 sensors?

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    It is illegal to sell used catalytic converters in California. I could not find a CARB compliant catalytic converter for the Prius, the shops even state that they will not send to California for legal reasons. My mechanic didn't check the O2 sensors, only the codes thrown by the OBD-II computer. My only option to the dealers would be to install a after-market one, but I'm concerned if it will pass the visual inspection. Do you know if any aftermarket cat can pass the visual inspection? – Gabriel Diego Dec 15 '15 at 1:12
  • Have the 02 sensors tested before changing the converter. If it needs replacement, an aftermarket converter that is approved for california will be OK. Just make sure to save the corresponding paperwork. – race fever Dec 15 '15 at 1:14
  • I will ask him to check the O2 sensors before committing to replace the cat. Are you from California or do you know thoroughly the CARB regulations? It is painful here when the car needs a new cat since there are many rules made just to make this job more difficult like no reused cats or no non-compliant cats, even if the car can pass a emissions test. – Gabriel Diego Dec 15 '15 at 1:18
  • No sir, I am not from California. However, I used to be a Toyota technician. I do know that as long as the aftermarket part is CARB complaint and you keep your paperwork no issue should arise. Mind you, finding a "friendly" smog inspection center will make your life much easier. Some are just scams that have you pay to pass even if your car is legal. – race fever Dec 15 '15 at 1:22

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