2013 Odyssey intermittent P0420

[dpilot83]

First time was probably at 15,000 miles. Stayed on for maybe 1000 miles before I could have it checked. Light went out either on the way to the dealer or the trip before that. They pulled the code from memory and said we probably had encountered bad gas since they couldn't get it to do it again.

Light came on for second time just a few days later and didn't stay on for more than a couple days.

I ordered the MaxiScan MS509 and hooked it up. The code in memory was again P0420 (says for bank 1).

I'm sitting here with the throttle at 2500RPM in park in the driveway. I am watching:

O2S11
O2S21
O2B1S2
O2B2S2

B2S2 changes but slowly. Sits at 0.6 for awhile, then goes to .595 for awhile, etc. B2S1 jumps around all the time. In the same time frame as bank 2, it may go from .575 to .610 and all over in that range multiple times. It just doesn't seem nearly as steady.

Could this be a problem with the O2 sensor itself or is that a sign of an actual bad cat? I can't imagine having a bad cat at 15,000 miles (now 19,255). There currently is no light but if it's happened twice I'm expecting it again. Any thoughts? Thanks.

 

[dparm]

P0420 is "catalyst system efficiency below threshold". The ECU is comparing the pre- and post-catalyst O2 sensors, and is seeing the post-catalyst sensor not registering reduced emissions (slight oversimplification, but it gets the point across).

If you clear it and it doesn't return, it's just OBD2 being fickle. If you clear it and it returns relatively soon, the cat converter is likely failing. Not saying it isn't a bad O2 sensor, but there are other codes for over/under-voltage.

Since the car is under warranty, let the dealer handle it. Emissions systems are covered for 8 years/80k. If the car is out of warranty, my advice is generally to inspect and clean both O2 sensors. I also recommend that people bang on the cat converter with their fist or a rubber mallet to listen for a loose catalyst brick. Sometimes the substrate will deteriorate over time and cause the catalyst itself to rattle around, which will of course give the light you describe.

 

[JHZR2]

Either could be the cause, but I agree with the above advice. Unless your vehicle has a voided warranty, take it to the dealer and let them determine it. Its part of the reason why you paid for a new car warranty when you bought the car (included in the price).

A converter could have been a dud, could have a bad washcoat, etc. Or, a connection on an O2 sensor could be bad. All no big deal, IMO. But you want it documented and fixed.

 

[The Critic]

It's probably a software issue. I would not be surprised if the proper fix involved a revised software calibration.

 

[dpilot83]

Unfortunately I'm not near a dealer often so in the meantime I'm trying to learn as much as I can.

I don't really intend to replace the cat myself if it is the cat, but I would be happy to switch the two post cat O2 sensors to see if the problem follows the O2 sensor or see if it stays the same. Would there be any merit to this diagnostics step?

When I do get to the dealer I would like to have enough information for them that the problem will actually get fixed while I'm there (which did not happen last time).

 

[dparm]

Originally Posted By: dpilot83
Unfortunately I'm not near a dealer often so in the meantime I'm trying to learn as much as I can.

I don't really intend to replace the cat myself if it is the cat, but I would be happy to switch the two post cat O2 sensors to see if the problem follows the O2 sensor or see if it stays the same. Would there be any merit to this diagnostics step?

When I do get to the dealer I would like to have enough information for them that the problem will actually get fixed while I'm there (which did not happen last time).



Don't take this personally, but I wouldn't start telling the service writer or dealer techs what to do. Let them diagnose it according to their procedures. You don't want to inadvertently give them incorrect information that stretches out the repair. The techs are also not going to be thrilled if they notice someone has already been under there. It's a warranty issue so it's not like they're going to take you for a ride and hit you up with some ridiculous bill. It's not in the dealer's best interest to half-[censored] the job since Honda won't fully reimburse them for sloppy work.

After the repairs are complete, THEN you can ask for the specifics.

BTW, swapping the sensors is often impossible due to different wiring lengths. On my old Integra, both sensors had wiring that went into the same hole on the floorpan, meaning that the post-cat sensor's wiring was very long and the pre-cat's was very short.

 

[webfors]

My P0420 went away when I started adding 2 stroke oil to the gas. Had the code 3 times in the past year, none in the past 5 months. Could very well be the cat is failing, but it hasn't returned since

 

[Roadkingnc]

If the car is under emissions warranty take it to the dealer, the catalyst monitor switching indicates a bad catalyst, it should remain steady and not switch like the front sensor.
It doesn't have to be restricted to be bad, some just go chemically inactive.