I have a 2005 Honda Pilot with 177k miles amd my CK ENG light is on and I'm getting a PO420 code. We're overdue for our Smog Check (CA) and this is driving me crazy! Hoping someone can give me some tips or help getting this light to go out without replacing all THREE Catalytic Converters.

(My smog guy says to drive the Pilot at 56-63 MPH for 15 minutes, 3 times and if the CK ENG light goes off, drive directly to him to get the vehicle passed.)

I did some research today and I was going to purchase a O2 Sensor and go one by one changing it out to see if this made a difference. I'm also thinking of getting a couple of Spark Plug Non Foulers to make space for each O2 Sensor. A tip I saw on a Youtube video. Before I go Easter Egg Hunting, I thought I'd ask a group that actually knows what they're doing first.

Any/All suggestions would be greatly appreciated!

 

Replace the cats. Well, that code is specifically for bank1 (rear of the engine) Replace that one at least.

If you buy defoulers you'll fail smog unless you have a veeeery good relationship with your smog guy.

 

P0420 is not a catalyst failure code.
It is a 'below threshold efficiency' code. 'Your engine is out of tune.'
If the engine has not had tuneup in a while, valves not adjusted, MAF(if applicable) not clean and various other tuneup issues will cause a failure.
O2s are a good start as well, these sensors can become lazy and if they are not switching as frequently they may cause A/F issues.
Hondas early PGM-FI OBDII system liked to chuck the P0420 even or an obvious misfire that should set a P0300 code.

Catalysts don't 'wear out'. Either it has been mechanically damaged from an impact, overheated from too much fuel, or chemically poisoned by ingesting a contaminant such as silicone or anti-freeze.
If the catalyst has failed you would be better at diagnosing why the catalyst failed and rectified that issue prior to purchasing a new catalyst.