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Q: Could it be anything else besides a bad cat? on 2006 Toyota Sequoia

Dreaded P0420 code. I've sprayed carb cleaner in the engine bay to test for air leaks, I've cleaned MAF sensor, replaced PC valve, no leaks in header or exhaust. Reading O2 sensor voltages produces these results. Under load both left and right post cat sensors read pretty similar. When coming off the gas both eventually drop to a lean condition. Then left post sensor almost immediately jumps to full rich. I don't think it’s a leaky injector because the left pre cat sensor doesn't spike. If anything it goes just a little lean on the left side (compared to right). I switched the post cat O2 sensors to see if it could be the sensor itself. No difference. I think it has to be a cat going (gone) bad. Does this all jive? Thanks a bunch for the help!!
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1. Verify that the fuel trims on both banks are good at idle and when driving the vehicle. If the fuel trim is not correct, check the Mass Air Flow (MAF) sensor. It is possible to have the fuel trim at 20% and not set a fuel trim code and cause the catalytic converter to not function properly.

2. If the fuel trim is only off on one bank, look for a faulty O2 sensor, a vacuum leak or a fuel delivery problem on the bank the fuel trim is off on.

3. If the fuel trim is good and within 10% plus or minus, drive the vehicle at a steady throttle, steady cruise and monitor the rear (B1S2) O2 sensor operation. If the rear O2 sensor is switching rich to lean, the catalytic converter is not storing oxygen correctly. When the catalytic converter functions correctly, the rear O2 sensor should not switch. If the rear O2 sensor switches at a steady cruise while driving, the catalytic converters are failing.
Tech Tips: On this vehicle, bank 1 is the driver's side bank.

Thanks very much for the input.

I’ve reconfirmed I have no top end air leaks and I cleaned the MAF sensor last week. I checked the long term fuel trims last night. At cold idle they are both (left/right bank) at about 10%. Once warmed up idle is at 7.64%. Under load they both look good too, always staying below 10%. Short term trims looked normal too.
Here is what I am seeing. Under load or setting the cruise control at 60 both left and right post cat O2 sensors match stay at a constant voltage. The left side is a bit richer than the right though. Once you let off the gas both fall to a lean condition. When you get back on the gas just a little bit the left side shoots straight to full rich condition. The right side takes a while to make its way back up to a richer value. Also, if you don’t get back on the gas the left will jump back to a full rich condition in about 3 seconds. The right takes a while to slowly rise to a mid rich condition.

Any other thoughts?
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