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Hi
Need some advice.

2007 2.4l 275K miles I've owned the car since 2010, 33K miles. I've had great luck with this car, no major issues ever, except for the brakes randomly going out on me about 2 years ago-which was fixed with a recall.

Not sure if any of this is related. I screwed up, timing belt broke in July. Had to replace the head. Lucky for me(at least financially)same model/year had wrecked, but engine was in perfect shape in tow lot behind the shop. Picked it up and drove it, it was louder. Seemed like it was coming from the exhaust and acceleration once at about 50mph seemed sluggish. Got the mechanic and owner in the car, said it was fine. Owner said well, totaled car got the head from had 80K miles, yours is almost 300K something with the valves, but it was fine. I watched the mechanic pull out the plugs, he said they were still good.

Few weeks later, oil light pops on. Then check engine light-I forget which order but was within about a week of each other. Just replaced the oil pressure switch, it was leaking, and oil light is off now. Pulled codes for engine light, PO 421 and 431. Emissions related, both of us thought O2 sensor. So as I'm searching around for parts, I see it may be the catalytic converter. Or plugs. Or exhaust leak. Or O2 sensors. I am losing about 50 mpg per tank. And as I said, my acceleration at highway speeds is now shot(hoping it's related to this), which sucks when trying to pass those jerks camped out doing 65 in the fast lane and want to speed up when you try to pass them. Only other issue prior to timing belt change, check engine light would pop on within 75 miles of gassing up, and shut off at about 250 miles. No issue with mpg, and I blamed that on the gas cap-someone stole it awhile back and I realized why, because they're hard to find around here for some reason.

I'm not able to check the O2 sensors myself. I can do basic stuff, like changing the plugs, oil changes. My shop...I trust them more than any other shop around here. But, I don't. Is it common for both of those codes to be thrown together? Quick search, seems it's both banks-haven't been able to determine if it's up/downstream yet. Was trying to avoid replacing all 4 O2 sensors since they're running about 98. each currently at the parts stores. Was thinking about trying RockAuto where they're half that cost. But now with both codes, I"m wondering if it's the converter that's bad? Any way to check that on my own with basic/limited knowledge? Just trying to make this car last 2 more years. I realize with the year and mileage, it was going to happen that I"d have to start forking out money on it. But just not a good time right now....but is it ever :)
 

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I threw those same codes a couple of years ago. I had to replace both O2 sensors on the bottom of the front exhaust. and I cleaned my mass airflow sensor with the specific spray**, changed the air filter, and used brake cleaner to clean the carburetor from all the grime and carbon build-up. Finally, I threw some seafoam at the end to clear out the system a bit. It worked for my 2009 gs

I think there was a thread on here that explains more about it just search forum. I know I had this one bookmarked back in 2018.



https://www.club4g.org/board/4g-eclipse-gs-se-specific/23576-help-codes-po421-431-a.html
 

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Thank you for replying. That link was interesting, sounds very similar even down to me just finding out there were 4 O2 sensors.... It dawned on me a little bit ago, I'm guessing they had to take off the O2 sensors when they changed the head out. I'm wondering if he put the wrong sensors on the wrong spot maybe? Would the car even drive if that was the case? And is it possible it would take a week or two for it throw the codes? I just keep thinking there's some connection when he had all that stuff off and then the light coming on soon after. I do get though that with the high mileage, they're probably long over due for replacement anyway.
 

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I threw those same codes a couple of years ago. I had to replace both O2 sensors on the bottom of the front exhaust. and I cleaned my mass airflow sensor with the specific spray**, changed the air filter, and used brake cleaner to clean the carburetor from all the grime and carbon build-up. Finally, I threw some seafoam at the end to clear out the system a bit. It worked for my 2009 gs

I think there was a thread on here that explains more about it just search forum. I know I had this one bookmarked back in 2018.



https://www.club4g.org/board/4g-eclipse-gs-se-specific/23576-help-codes-po421-431-a.html
We don’t have carburetor’s haha u gave me a good laugh, I assume you mean the throttle body
 
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