I've had a Cat plug up. It was on an 83 F-150. This truck had the variable venturi carb and would DUMP fuel. You could feel the performance degrade, and the last trip it took with the plugged Cat, it wouldn't do 25 with your foot on the floor. We limped it back home, and to test, pulled the exhaust to make sure (this was in the days before all the sensors and computers, so no O2 sensors to worry about). Truck fired up and ran like it should. Fixed the carb issues and the exhaust..... and truck ran fine for a long time..... until it chucked a wrist pin (but that's another story and not related to the Cat).
I'd say it if runs fine and BAM, the CEL comes on and now it's in "limp mode" it's "electrical", not a bad cat. They don't just come and go, they plug and then you are on the side of the road. Was the replacement O2 sensor an OEM one? I have a transient CEL for an O2 sensor on my 07 Infiniti QX56 (no limp mode, caused by a tiny exhaust leak). One thing I've read over and over on posts about O2 sensors is only buy OEM Name Brand O2 sensors. The imports seem to be really sensitive to aftermarket O2 (well most any engine) sensors. I know many of the newer vehicles don't like aftermarket sensors (my 6.0 gets ONLY Ford/Motorcraft electronics).
My opinion is, it runs fine, no performance degradation, and CEL light comes on and now your fuel economy tanks, you have a sensor or wiring issue. The PCM isn't seeing what it expects (in this case, it thinks the engine is running really lean, it richens the air/fuel mixture trying to get the sensor reading within spec (which it can't), it throws a CEL and starts eating fuel.......
Marlin, another thing to check for is exhaust leaks upstream from the catalytic converters. How old is the air filter? Here's some good info:
Addressing some common misnomers of the P0420 code
Thank YOU. I will be calling them. My son has given some suggestions. Apparently those codes can be pretty generic.Here is a thought on the Tacoma. Our daughter lives in the San Fran Bay Area. She has a Prius. I have not met this person but she has a mechanic that seem to be very good. He and his son operate a very small garage. They only work and Toyota brand. Our daughter really likes him and he seems very good, fair and knowledgeable. My experience with young women taking a car in for repair is that they are often taken advantages of. In the reviews people actually say that they buy Toyotas just so this person will service them. My guess is that he will talk to you via phone and provide solid advise. It not an add, I have not met the gentleman. Here is the link: Toy Auto Clinic Toyota Lexus Scion Specialists