P0420 CEL - Why diagnosis is key

So my coworker has an '06 GenII with 198k miles on the odometer that she noted had loss of power and CEL illumination the other day. She asked me to take a look at the car, so I went home and grabbed by scan tool and came back to the office to look at the car.

When I started the vehicle, and the ICE turned on (in closed loop) and the engine was missing slightly/idling rough. My scan tool showed that the ECU was throwing a P0420 - Catalyst System Efficiency Below Threshold (Bank 1). This suggested to me that the vehicle was running rich (did not check LTFT or STFT because scan tool was not working properly since my paid app kept crashing and I only had the ability to pull the code) and the catalytic converter was unable to burn off the high level of hydrocarbons in exhaust gas. I looked through maintenance history of the vehicle and there was no record of the spark plugs being replaced; therefore, I suggested to try and replace the plugs as a first pass since there were no misfire codes (this would suggest a bad ignition coil).

The spark plugs were removed (NGK IFR5A11 original plugs? [superseded by NGK 4996 - #IFR5T11 in 2010] - quite worn and had quite a bit of white deposits and carbon build-up) and replaced with Denso 3324 [SK16R11] - (a small amount of Anti-Seize [Aluminum, Permatex] was applied to the threads of each plug) and torqued to spec (13 ft-lbs). The ignition coil O-rings were cleaned of debris and Permatex Dielectric Tune-Up Grease was applied sparingly to coil boot and spread with a small brush. The coils were reinserted and bolt was torqued to spec (80 in-lbs). The throttle body was cleaned of carbon using Valvoline Carb and Throttle Body Cleaner, Hoppe's No. 9 Gun Bore Cleaner (this is the best carbon remover I have found), a tooth brush, and a rag. The MAF sensor was removed, the O-ring was cleaned of debris, cleaned with CRC Mass Air Flow (MAF) Sensor Cleaner, and reinstalled.

When the car was started, there was no missing/rough idle. The car was next taken on a 10 - 15 mi test drive, including highway, and floored a few times. The LTFT was steady at 0 and -0.78% and STFT was trending +5% and the vehicle seemed to have great power and acceleration. The efficiency went from 43.1 to 44.0 mpg on the HUD in this short trip.

The vehicle doesn't seem to be an oil burner (about 5k on the current change and dipstick was at full) and has a pretty solid maintenance history (mostly dealership). What is absolutely nuts is the fact that in 2015 @ 120k miles, the dealership serviced the vehicle and they literally did everything, but didn't report replacing the plugs... I am convinced that these plugs are the originals and have almost 200k miles on them. Goes to show you how good NGK and Denso plugs are.