The goal is to learn how to diagnose a P0421 using an OBDII scanner.

Request for diagnostic advice to pinpoint least expensive solution
to resolve persistent P0421 OBDII DTC given...

1. College kid's early 2000s Mitsubishi fails OBD-only smog
2. Neighbor asks me to help out (as if I know anything, which I don't)
3. However, I can borrow the vehicle & run live tests via OBDII scanner

Question is, what do I look for in terms of "live data" to help diagnose?

DETAILS FOLLOW:
o 180K miles, circa 2003 or 2004 model year (I think)
o Service Engine Soon has been reported to be on for a year
o New pre cat 02 sensor had reportedly already been installed 3 months ago
o Lambda O2 Sensor always tests OK on the OBDII scanner
o CAT always tests OK on the OBDII scanner
o All other registers always test OK on the OBDII scanner
o The P0421 DTC is set "sometimes" when the CAT register is set
o I cleared the DTCs a few times but the P0421 came back each time
o I/M Readiness lasts only about a day or three before DTC occurs again
o Always the same DTC and always only that single DTC
o Warm Up Catalyst Efficiency Below Threshold Bank 1

The question isn't "Does it require a new cat", nor is the question
"What parts can we blindly throw at it until it finally works".

The question is what "live data" do we look for to _diagnose_
what the problem actually is (and what's the options for repair).

My main dilemma that I'm asking for your help on is what
do I look for in the "live data" when I take it for a test
drive with the OBD scanner?

BTW, I'm familiar with the concept of the Federal Test Procedure
(also known as the Drive Cycle), but I don't have the specific
drive cycle for this specific vehicle (but I can guess based on
the drive cycle for other vehicles, although not the same marquee).

PHOTOS FOLLOW:
o <https://i.postimg.cc/KzTmdCBx/obd01.jpg> Failed smog
o <https://i.postimg.cc/nrYpL73W/obd02.jpg> 180,000 miles
o <https://i.postimg.cc/65tBmj3x/obd03.jpg> 1 DTC with all registers set
o <https://i.postimg.cc/fRbDwVmH/obd04.jpg> P0421 below threshold, bank 1
o <https://i.postimg.cc/437G9j8V/obd05.jpg> Lambda sensor OK
o <https://i.postimg.cc/kgh9D7y7/obd06.jpg> CAT OK
o <https://i.postimg.cc/901KSB8c/obd08.jpg> EVAP, MIS, FUEL, OK
o <https://i.postimg.cc/26nfHhN5/obd07.jpg> Live data page 1
o <https://i.postimg.cc/v8vJcQQC/obd09.jpg> Live data page 2
o <https://i.postimg.cc/vHqCt3xz/obd10.jpg> Live data page 3
o <https://i.postimg.cc/NjKSJm1w/obd11.jpg> Live data page 4
o <https://i.postimg.cc/T3BSZ195/obd12.jpg> What values do I look for?

QUESTION:
o What do I look for in terms of "live data" to help diagnose?
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The goal is to learn how to diagnose a P0421 using an OBDII scanner.