Code P0420

This truck is a 1999 Tacoma 4x4 2.7L, 3RZ-FE 5-speed. 165K miles.
I can't get rid of the CEL for this code.
I have carefully installed brand new up stream and down stream 02 sensors that had scanned bad using my OBD II scanner. The codes for them no longer display after replacing them.
Have also carefully cleaned the MAF and cleaned out the throttle body.
I used the correct cleaners for both of them and did not touch the MAF at all.
Once I did clean the MAF the CEL stayed off for about 3 days but is now back with the P0420 code again.
I'm wondering if the CEL now is some other sensor now after resetting the PCM by disconnecting the negative battery cable for 5 minutes.
What other sensors can I check?
I should mention, the scanner indicates a "?" mark next to the downstream 02 sensor when I did the 02 monitor test and when I look this up the scanner "cannot retrieve data" for it.
Do I have to check the 02 sensors after getting the engine good and hot though? I did not have it hot when checking them.
My goal is to find out for sure if my catalyst is failing before spending that much money on one.
All fluids and gas cap are fine. Cannot see or feel an exhaust leak but could there be a very small exhaust leak causing this problem?