[2011.2017.odyssey]

I only use shell gas for the fleet.

Got the P0420 code a month or so ago and ran a cup of acetone through a half tank of gas, did this procedure twice.

Had the old gal up in the air to do the strut rebuild last weekend, and noticed the flange bolts to the bank (1) cat where it joins the downpipe were disintegrated and hanging on by rust.

slight signs of exhaust escape around the flange but no noise, so heated up the rotted bolts air chiselled the rust off and tapped them out.

Replaced with new hardware.

I think this was the original cause of the P0420, with air getting in and throwing the readings off.

if you think your cat could be starting to go, acetone doesn't hurt with proper dilution.

There are some vidyas on loobtube that show removing the O2 sensor and getting a borescope/endoscope camera in the hole to see if the honeycomb element is coming apart.

a sure sign the cat is done and needs changing.

 

[2011.2017.odyssey]

probably, the downstream oxygen (O2) sensor (190K miles on it).

Have you tested each O2 sensor?
Your foxwell graph isn't much help... the voltage data is captured with ECT1 at 158F and above running temp, and engine rpm at 3000 rpm without a load.
You should be seeing 1.27 volts, based on the above test conditions.

Heated O2 Sensors (HO2S) also have a 90k mile service life (right out of the crv service manual)
Anything after that is borrowed time.

Like mentioned before, why don't you take the cat O2 sensor out and get a borescope camera in there to look at the element.
Going to wager it's starting to come apart in pieces.

If the cat is not coming apart, then try a few applications of a cup of pure acetone in the half a tank of gas, that would be my next try.

Have done it on theody a total of 6 times now, no issues and no P420 anymore.
Original cats original O2 sensors at about 140k miles.
The O2 sensor heating elements are getting tired, she runs rich when cold but no codes.

 

[2011.2017.odyssey]

you think any of this could be gremlins coming back to haunt from the head gasket job?

 

[2011.2017.odyssey]

It might be temperature related, you posted this in October its now February.

Maybe the colder months might be masking this isue?

Inversely my old g2 ody the p420 would come on in the colder months not warmer, when it would run rich at cold start.
would clear the light and it wouldn't come back until the next winter.

 

[2011.2017.odyssey]

P0420 just popped up on my ody again last week, put another cup of acetone in the tank, and this weekend I pulled out the bank 1 upstream and downstream O2 sensors, got a borescope inside the O2 sensor holes and the cat element is gleaming shiny clean no breaks or holes in the element.

Cat is definitely good.

Soaked both O2 sensors in some injector cleaner, wire brushed some of the clogged holes, compressed aired them dry and reinstalled.

Ordered 2 NTK's off rockauto to replace.
voltages look good for upstream and downstream sensors, but one could be intermittently flaking out.

Question...

I don't see any follow up above on your O2 voltage or test results based on post #8... you need to zero in on that one.
what is your cat temps enter and exit? use an IR temp reader.

 

[2011.2017.odyssey]

O2 sensors. Defferent people call them differently. All I understand is upstream and downstream.

The service manual will call them HO2S sensor 1 (primary / upstream) HO2S sensor 2 (secondary / downstream) Or Bank 1 Sensor 1/2 or Bank 2 Sensor 1/2

Your CRV "should" have 2... The ody has 4, 2 per cat.

 

[2011.2017.odyssey]

I lost about less than half a gallon in 1.5 years.

time for a plug inspection.

if they are white and dusty looking then its eating coolant, otherwise since the gal is up there in years it could be rad a or weak spring clamps venting coolant.

my ody was doing that last year, and was slowly losing coolant.

 

[2011.2017.odyssey]

seems to be a lot of crap caught up in the element. did you just apply the lacquer and what were seeing is that?

 

[2011.2017.odyssey]

Some stuff caught up in the element is normal, but that looks excessive.

Maybe it's a leftover from burning all that coolant from the head gasket issue?

You can also see that the element is also crumbling away in the center.

you could try saving the the thing by taking it off and soaking/rinsing it, but you're likely better off just changing it while it's off.

This is what the original bank 1 cat looked like on my 2011 ody, after changing the upstream and downstream O2 sensors in an attempt to rectify a P0420.

As you can see there is also some crap caught up in the element.

That cat burned off lots of oil for the VCM piston ring issue before it was fixed under TSB then muzzled at around 60k miles.

The resolution on my cheapo borescope isn't the greatest compared to yours.
some liquid in the second photo to contrast the element.

 

[2011.2017.odyssey]

When was your cat inspected? what mileage then?

That photo was taken a few weeks ago, the mileage is around 130,000+ miles.

Van has new NTK upstream and downstream O2 sensors now on bank 1.
if the P0420 comes back its certainly the catalytic converter and will swap it out with a EPA compliant davico or ap/eastern cat off of rockauto.

went with the "cheap" items first.