from a quick google search it looks like catalytic converter issue or exhaust issue in general. maybe someone will reply with the troubleshooting steps for that code
It's cat below threshold downstream 02 possibly I've had this on 2 different cats and then on a replacement new unit . Same cat installed in my Corolla yields no mil light. The aftermarket brick type weld in cat covers Corolla Prius gen2 and others under 5K pounds . Seems to be a sensor or ECU issue . Stills passes smog tailpipe test in Mass. Fails only because lites on . I've had a rider on it before . SM-A715F ?
The Engine Control Module uses two sensors to run a test to check how well the catalytic converter functions. These sensors are the Air Fuel sensor (it's upstream or in front of the converter) and the oxygen sensor (this is downstream or after the converter). If the ECM determines that the converter doesn't work as well as it should, then it will set a code P0420 for low converter efficiency. So this code could result from the converter being degraded, a fault with either sensor, or an exhaust leak near one of the sensors. A poorly running engine (esp a misfire) can damage the converter. If the exhaust system has NO leaks anywhere near the sensors, AND the sensors and engine are tested and found to be functioning properly, then you will likely have to have the converter replaced. Posted via the PriusChat mobile app.
I've been around and around with this code I had an 05 the converter was stolen or removed and sold The O2 sensor was unplugged nothing cut nothing damaged they even unbolted the converter no cutting prior to this the car had no codes was a 49 mile to the gallon car nothing I bought a catalytic converter from the big place on eBay they make cats for all kinds of cars they pass in every state The only reason they can't be sold in California is because they are not from the original equipment manufacturer but if you connect them and you take them to an exhaust gas analyzer everything is within spec everything is good on the exhaust gas analyzer The car I have the p0420 code and it never can go away I don't know what the cat being below threshold exactly means I replaced my catalytic converter with the brick type not the fine wire type exactly like what was in the car My converter that I purchased side-by-side to my factory original converter is actually pretty difficult to tell apart other than the stampings on the number plate so I don't even think the dealer can clear and fix this and I haven't even attempted because everything in the car has been replaced My next move if I even decide to take it will be to gut the catalytic converter empty out of brick cut off the silencer put it all back in the car like my 09 is now and I still have the code in the '09 so not much to say SM-A715F ?
My factory cat had a P0420. Turned out to be the donut gasket upstream from the converter was worn out. Recommendations were to replace with factory gasket $50. Before I got a chance to do that the cat was stolen and I wound up replacing the entire cat with one from ebay for around $150 (gasket included). No CEL since. Has been 6 months and many miles.
That's the one I have right off of eBay My buddy that bolted it in for me while I was standing there because he's got a lift I mean we can stand under the car and breathe while it's running and hold the hand over the tailpipe and hear nothing when you let go is a big whoosh and that's about it so maybe it's one of my O2 sensors I kind of doubt it but got nothing better to do throw midline parts at the thing and see what happens any of these parts I buy that don't do the trick I can just put them on one of my other cars so I don't know they're all replaced so it's not a total waste SM-A715F ?
if your cat was stolen and your O2 sensor was also taken, did you replace the rear sensor completely? Harness connector to sensor or splice in a new sensor? I thought there were potential issues with soldering O2 sensor wires, but can't remember the details. Replacing the entire sensor with a quality brand would be safest.
