The Infamous NB P0420 Code

I have read the other threads on the forum about this, but I've replaced pretty much everything recommended besides the coilpacks.

I have a 2000 Miata with 70k miles and I have been getting P0420 codes on and off over the past few months (mainly appearing when it's cold out). I replaced the front 02 sensor with a new Bosch unit and at the same time replaced the spark plug wires and spark plugs with NGK parts which got rid of the code until just recently.

I've always had a rough idle (warm or cold) and lower power when cold (very noticeable at 3k rpm). I also noticed a gas smell in the cabin (but not in the trunk where most people have it), but I'm not sure that's related. Since the code has reappeared, I'm thinking it's time to replace the coilpacks, but I'm curious to know if I should replace anything else. I don't think it's the cat because the midpipe is much newer and cleaner looking than the rest of the exhaust system (I'm assuming the PO replaced it).

The code disappeared for a few weeks and during our freak ice storm it returned. Of course I start my car today and the CEL is gone, but it's warmer out. I still have the somewhat rough idle though.

Instead of spending $300 on coilpacks and hoping it fixes these issues, maybe I could meet up with someone locally and swap coilpacks to see if it fixes the rough idle issue.

I believe gabkwong had (has) the same issue on his 99. Maybe he will chime in on your post. I know he did a lot of things to his car to get rid of the code.

If you do have a bad coil pack and it's misfiring, that will kill your cat too.

Did you clean the EGR port?

P0420, or did you mean P0402?

When mine came up with P0402 it turned out to the the Cat.

ETA: I had the same symptoms in regards to it coming on. Sometimes it would, sometimes it wouldn't. Never had the gas smell or the idle issues though.

Quote:

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Originally Posted by sammm

If you do have a bad coil pack and it's misfiring, that will kill your cat too.

Did you clean the EGR port?

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I have not cleaned the EGR port, I've also thought about checking the charcoal canister.

Quote:

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Originally Posted by Rebel

P0420, or did you mean P0402?

When mine came up with P0402 it turned out to the the Cat.

ETA: I had the same symptoms in regards to it coming on. Sometimes it would, sometimes it wouldn't. Never had the gas smell or the idle issues though.

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Yep, definitely P0420. I've had it checked two separate times when it popped up.

About the gas smell, I've noticed it only occurs after I drive to or from work (20-30 minutes on the highway) and it's only in the cabin (I've checked the trunk as soon as I park the car). Sometimes it smells like burning rubber as well. All the hoses and belts look to be fine. I plan on changing the fuel filter shortly and I'll look into the EGR cleaning. Otherwise looks like it's time to poney up for coilpacks.

I'd do the EGR cleaning before spending money on replacing stuff. That did the trick in getting rid of my CEL when I had it a month ago.

EDIT: as far as the gas smell, does it smell worse in the trunk? I'd check the fuel lines there for leaks.

I believe possibly your CAT is RIP! It is possible that the O2 sensor died and killed the CAT while you were still driving it with CEL on. I did the O2 sensors changes and CAT. I did what you just did. Changes the O2 sensor and only bought several months and the CEL returned later. My major problem was the new high flow CAT only lasted several months and I got the CEL back again. I finally gave up and did the cheater fix. If you are interested on the cheater fix, PM me.

According to the 99 Shop Manual, P0420 can be one of the following:
1. CAT deterioration
2. Leakage exhaust system
3. Front O2 sensor malfunction
4. Rear O2 sensor malfunction
5. MAF malfunction
6. Throttle position sensor malfunction
7. Vehicle speed sensor malfunction.