Toyota’s have a troubled history with the PO420 code and I think you are in the beginning stages of this. You may not see the code because unless it repeats itself in subsequent drives, it gets deleted. It could be occurring often enough to block the monitor from being set, but not enough to set the CEL. You would have to catch the code right after it occurred.
Most of the time, it is not the cat. Exhaust leaks are one of the primary causes so look for one of these first. Next would be the rear O2 sensor, but it is best to replace this with one from a dealer based on the cars VIN number. You will pay more but it is worth it. If you can find someone with the equipment to monitor both O2 sensors at the same time, you can get a clearer picture of what is going on.
Do you have access to a scan tool? If you do after you clear the codes keep it in the car and look for when the monitors complete and if you have pending codes at the ends of any trips. I would also look at your fuel trims, long term should be around 10-12. Anything above 17 is abnormal and is the borderline where codes start to set. The O2 sensors can be watched for with a scanner but most low cost ones do not have high enough resolution to accurately monitor switching if one is borderline. If one is lazy you may be able to see it with a low cost scanner. The early years of obd2 the thresholds where codes were set were very liberal.
Update: I followed the Cataclean instructions, which involves running the tank down to 1/4 full, adding the fluid, driving 10-15 miles, then filling up again. Then I ran the prescribed drive pattern and rechecked. The Cat monitor was still not set, and I began pricing O2 sensors.
But I decided to hold off until the next weekend, partially because today I had a 50 mile trip up the freeway and back with start and end segments of 5 miles of primary streets, which sort of fits the drive cycle pattern except with longer high-speed segments. I connected up the OBD II scanner after I got home, and now the Cat monitor is set & no error codes! Tomorrow, I’ll try the smog check.
I couldn’t make it in to the smog check last week. I rechecked with my OBD II scan tool on Friday; all monitors set and no error codes, so I took it in for the test and passed. Yay!
The new standard California test for 2000 and newer cars only gives pass/fail status, not a readout on emission levels; I kind of miss that extra information.
I replaced the front 02 sensor 2 years ago for the last smog test; I think Kieth may be right and I’ll probably need to replace the downstream sensor for the next test.
I have this type of problems before. If you’re cat not ready mean that your ignition coils, spark plugs, wires are not functioning 100 percent like it should. It’s not bad to throwing code but not good enough to burning your gas completely that why your cat never get ready. Hope my pass problem help your future solutions. I changed mine and I drove it 50 miles in the free way it ready and pass smog not problem.