2012 Toyota Corolla downstream O2 sensor and Catalyst are not testing

What is your question?

1.8L 4-cyclinder engine. Downstream O2 sensor is partially testing. It completes part of the tests and before completing all 5, stops at the third or fourth and starts untesting part of what tested to go back down to 2 or 3 tested again. No OBD2 codes. Downstream O2 is getting voltage, O2 heater is testing fine. Upstream and downstream O2 are talking to each other when driving and fuel trims are being adjusted. 3 months ago, new iridium spark plugs and new air filter. Oil is clean and changed per mileage. We had this problem last year and somehow a Toyota tech got it to pass, but they're no longer there and didn't log what they did. The downstream O2 sensor was changed 3 times in the past 2 years (Toyota thought it was bad at 2 dealerships). New battery last year. Car has 115K miles on it. It's 10 years old. Could it be the catalytic converter itself?

What makes this problem better or worse?

Nothing

How long has your 2012 Toyota Corolla had this problem?

2 years