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Discussion Starter #1 (Edited)
2003 model. Got em both at the same time. Oddly enough right after replacing the alternator. May just be coincidental. Since it's the sensor at both ends of the cat, I'm thinking that ignition is a problem. It seems to be running fine, but it's been quite a while since the plugs were changed. Think that might be looking in the right direction?
 

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That's actually the same code for the front bank (3 cylinders) and the rear bank cats. Yeah, I'd say it's pretty unlikely that both cats decided to die on the same day. If it was my car, I'd clear the codes and drive the car for a couple weeks and see what happens. I'm betting it's just a glitch from having the battery disconnected for a while.

You might have to clear the codes a couple times, and it might take a few days worth of driving to get everything back to normal, but I'll bet it'll settle out soon enough.
 

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Discussion Starter #3 (Edited)
That's actually the same code for the front bank (3 cylinders) and the rear bank cats. Yeah, I'd say it's pretty unlikely that both cats decided to die on the same day. If it was my car, I'd clear the codes and drive the car for a couple weeks and see what happens. I'm betting it's just a glitch from having the battery disconnected for a while.

You might have to clear the codes a couple times, and it might take a few days worth of driving to get everything back to normal, but I'll bet it'll settle out soon enough.
Thanks. Cleared the codes by the battery ground disconnect method and let it idle for about 5 minutes. They did not come back. At least yet anyway. Hope that they don't as this week is the week that I must renew the plates and pass an emissions inspection here in GA. If you got a check engine light on, you can forget passing it. You can bet that the emissions station will be my first stop in the morning before I run a full drive cycle. I'll test drive it later. Still gonna change those plugs though. They have a lot more miles on them than they should have.
 

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Thanks. Cleared the codes by the battery ground disconnect method and let it idle for about 5 minutes. They did not come back. At least yet anyway. Hope that they don't as this week is the week that I must renew the plates and pass an emissions inspection here in GA. If you got a check engine light on, you can forget passing it. You can bet that the emissions station will be my first stop in the morning before I run a full drive cycle. I'll test drive it later. Still gonna change those plugs though. They have a lot more miles on them than they should have.
Update: came back today. Was told by a guy who spent a dozen years working for Acura that they may be legit since I've been a tightwad and have been using 87 octane. Gonna do a little more work on it tomorrow and if I can't figure it out, off to my trusted mechanic. Time's getting short before Saturday.
 

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I'm not sure that regular gas will have a dire effect on the cats, but I'm still thinking it's not the cats. Yeah, they could have both been RIGHT at the edge of failing, but something to do with the battery being down pushed the "system" over the edge (whether by losing some fuel system parameters that's causing the engine to run rich, or something).

FWIW, I've seen some additives that claim to be effective at "cleaning" cats. If your problem ends up being that they're both marginal, it might help (at least might do enough good to get you through emissions testing, which is what really matters). The other thing that can help is a healthy application of the loud pedal - the heat generated in the cats when you're running at full throttle can help burn off some gunk, too.

And also FWIW, I regularly ran regular on long (!) road trips on my '04 MDX. I'd always use premium around town or in the mountains, but for those long, flat interstate stretches, I didn't see a difference (I was monitoring the timing advance with my Torque Pro app, and it was the same for premium and regular on the flat interstate stretches). Never had a hiccup with my emissions system, though I did replace my upstream O2 sensors in an attempt to get better gas mileage (might have helped a little, but the big improvement came when I put a new paper element air filter in, replacing the "high performance cleanable filter" that the previous owner had installed).
 

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Got the p0420 code several years back and it turned out to be the rear cat. However, I recall the factory service manual stating that poor quality fuel (not octane) could be a cause. I tried Techron and a water remover called HEET in the tank. Didn’t work but was a inexpensive way to troubleshoot.

Here’s another post that may be relevant.


Good luck!
 

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I get that error every time the weather changes from winter to summer and reverse. The code disappears on it's own after a week or so of driving daily.

If you do need a new cat, at least the cats for the mdx are not that expensive around $800 I believe, many other brands are well over 1k, my old Audi was over 1600 each
 

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I wonder if the "seasonal code" might be due to a change in the gasoline formulation (some states switch over from a "winter blend" to a "summer blend", and I suppose that could have an effect on the performance of a cat, particularly one that's marginal to start with.
 
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