Once again I call upon the collective knowledge of this great community as I have in the past and have been very pleased every time. Today I drove my 2004 GT MT out of storage from sitting for several months. During those months I would start it and let it run for a while every few weeks or so. But today I took it out on a decent ride and it threw a check engine light P0139 which I have never had before. Last year during the summer my car would randomly throw a P0420 and It would go away on it's own. This would happen randomly and no noticible problems at all would happen. This went on for months and since there was no noticeable running problems I figured it was a bad sensor and not a bad cat. But now I don't know what to think, anyone have similar experiences with this combination of codes? Are these signs of a bad cat? I'm just over 70k on it and I do regular up keep on it. Any help would be greatly appreciated. Thank you all!

 

Also, I have no mods at all on my car. It is all stock and I use conventional quaker state 5w30.

 

Possibly from sitting and starting a few times during storage it got gummed up with gas and carbon. I would think it wants a good hot high rev pull to clean it out.

 

I have a midpipe(i.e. no cat) and my car throws p0420(cat is bad or going bad) and p0139(slow response by rear O2 sensor) when it figures out theres a problem down there. So something is amiss when a car with a cat is throwing those.

You may want to drop your cat and inspect it at the very least. Have you ever run your car hard and then got out and looked under it to see if the cat was glowing? You may be right and its only that O2 sensor, you could start at this thread. https://www.rx8club.com/series-i-trouble-shooting-95/p0139-during-hot-weather-171942/

 

Thanks xexok and godesshunter for responding, I have never gotten out to look at the cat if it was red hot or not, should it be red hot after driving it hard or not?

Talked to a dealership today about the warranty covering the cat and it looks like I'm just under the 8 year mark by 30 days or less and I'm not at 80k yet so I plan on taking it in and having them do a full check on it costing $105 but if it is indeed a bad cat that cost will be free. Not sure if the warranty covers the sensor or not but at this point I'm almost hoping the the cat is bad so I'll get a brand new one lol. Anything I should make sure these guys do? I've never had this dealership do any mechanical work on my car so I don't know how good they are, I'm always leary of places at first just from hearing all the stories about crappy service at some mazda dealerships. I'll be taking it in to Stamford, CT Mazda, it's the closest one to me.

 

Core and cat warranty don't cover sensors.
If indeed is the cat, make sure the dealer refund the $105 to you...

 

Thanks, I was thinking that. He did also say if the cat is bad the $105 will be covered by them which is cool.

 

Sorry I should have mentioned that, if it is glowing that is a bad sign. I hope its your cat also so you can get a free one.