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Return of P0420 code?

Please note, I'm not car savvy, including lingo, nor can I really fix stuff myself when it comes to cars. So keep that in mind!

2008 Volvo S40 with 219,816 miles. Beginning in 07/2020, I began having a check engine light come on and off that was identified to be code P0420. In 03/2021, I finally got it fixed when my local mechanic replaced our catalytic converter. Looking at that invoice, they also replaced the exhaust gasket, upstream and downstream oxygen sensors and replaced spark plugs.

Fast forward to now. On 2/10/23, my wife mentioned our check engine light came on and it's still on as of this message. This past 2/14/23, I took it to Autozone to get the code read and he said it showed code P0420. The rep there asked if we got gas recently and I told him yes, my wife got it the previous day on 2/9/23. He recommended simply changing out the gas cap to fix the problem. This is interesting to me because all that time from 7/2020 - 03/2021 when I finally replaced the CAT, I had always suspected and wondered if it was simply a gas cap issue. But I read information online with others shooting this idea down and say it would show a different code for that, etc. Is this simply an easy fix with the gas cap since we just replaced the CAT, etc. just two years ago? But even if it is just the gas cap, did I replace the CAT, etc. for no reason at all? My wife has mentioned the car doesn't feel any different driving since the light came on, from the best she can tell.

If this has nothing to do with the gas cap, I'm so lost and have no idea how to fix this problem. So confused and frustrated.

Thanks in advance for any help you can give.

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@bd3616 for now, go ahead and replace the gas cap. It is absolutely possible that this trips the P0420 code (more common would be a code for a leak in the EVAP system. Those would be P0442, P0443, P0446 for your Volvo) after a while. It is a pretty generic code with lots of possibilities. It is possible that you CAT was worn out but it appears they threw everything but the kitchen sink into that repair and replaced all of the parts that could cause that code. We really can't second guess your mechanic on that since we just simply don't know. Personally, I have not worn out a CAT even with all my high mileage vehicles. Lot's of O2 sensors and lots of gas caps, but never a Catalytic Converter. After 2 years (you did not tell us how many miles) it should not have worn out again. Your O2 sensors are usually good for something like 100k miles as well. Anything that messes with the Fuel/air ratio can cause this. Something like a faulty Fuel Pressure Sensor etc. must be considered.

Let us know what happens when you change the gas cap and drive a few miles :-) Best of luck

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@oldturkey03 thank you for your reply. Sorry, I forgot to include the mileage since we replaced the CAT in 03/21. It's been 35,584 miles since this P0420 came back up two weeks ago. Also, if it helps according to my notes, my mechanic used a Bosal brand CAT that has now changed names to BRExhaust. Our mechanic told me that it is still considered OEM grade though and that it's almost exactly Magnaflow minus the sticker. They couldn't do Magnaflow for $400 more because they said it was a different bolt pattern for our car.

So you still think it's a high chance of just the gas cap even though it's not a code for leak in EVAP? My wife has mentioned that at times when she fills up gas, the cap seems loose when she is almost positive she tightened it pretty good when last getting gas. We also always get mid-grade gas after replacing the CAT two years ago. If I replace the gas cap, will the code shut off by itself if that's the problem? Because I don't know how to reset the code nor do I feel comfortable trying if it's tough. Also, is it hard to replace the gas cap or pretty simple?

https://www.amazon.com/gp/product/B00D5L...

Does this one work?

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@bd3616 I am certain that your CAT is good. So that cost will not come at ou. The code gets send from the downstream O2 sensor so if anything,there maybe something going on with that. Ultimately, even that should still be good. MAke sure that all your Vacuum lines and still good and you have no exhaust leak, TRy a couple a tank full with supreme fuel to see if that helps. Do I think that it is the gas cap for sure? No, not as a primary reason. But there is always thing that can have consequences and the error code that you get might be a consequence of a leak in the EVAP system. That cap will work and it is easy to replace. The hardest part will be to connect the holder through the little hole in the filler cap opening. It'll take you all but 5 minutes. The code most likely would reset when you drive your Volvo for a while. You could also try Autozone after and see if they can use their OBD tester to clear your codes.

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@oldturkey03 thanks again for your help. Unfortunately, all my research for this code online is saying it would have nothing to do with the gas cap and others are dissuading me from going up that alley...I checked this Amazon part and they said if it's used, I wouldn't get a refund back so it would be $40 down the drain anyways since I couldn't return it. I wish it was just this gas cap but sadly it sounds like it won't be so maybe I shouldn't even attempt it? In the past couple of years, we have also noticed lack of acceleration in building MPH when accelerating up hills, merging on to the interstate, etc. Others online are saying due to this it could be another indication of a CAT issue and again another reason for it not being the gas cap. Also, I'm not sure of the seriousness of this check engine light? Like with this P0420 code....is this causing serious harm with me not getting it fixed right away? Is it ok to be driving on it? I'm afraid even attempting to see if a gas cap is the issue would take more time for this to worsen rather than just going to a shop?

Just so frustrated because I don't understand why this code would be coming up again after just two years. No idea how to proceed.

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@bd3616 I totally understand your frustration. You gotta start somewhere so let's agree to not change the gas cap since that would not be the right code. Now, we are back to square one. CAT or not. You can have a shop test the temperature before and after that CAT after it reaches proper engine temperature. So, like running it for 10-15 min then measure the temps with an infrared thermometer. If the temp is significantly higher pre than post you know it is clogged and exhaust gases are not passing through. You can also check this with a vacuum gauge. Connect that to a direct intake vacuum line and at 2500-3000 RPM you should have a vacuum of 15-20. Now increase the RPM and your vacuum should drop with a quick return to that same number. If the vacuum stays low, you have to high a back pressure, which points to the CAT. Check for Vacuum leaks and exhaust leaks as well. This is important, since something as minute as a split vacuum line, can affect your air fuel mix ratio and give you that code. Yes, this"lack of acceleration in building MPH when accelerating up hills, merging on to the interstate" could be CAT issue but you get those with other issues as well.

I wish we could just tell you "it's this, change it and it'll be okay" but sadly, that is not possible in this case.

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Check out some of the posts on the subject of P0420 on Matthew's Volvo Site, which has been an invaluable resource for me and my Volvos.

The P0420 trouble code officially translates to "catalyst system efficiency is below threshold (Bank 1)", meaning there's not enough of a difference between the reading of the front oxygen sensor (before the catalyst) and the rear one after it. You've already done the obvious fix in replacing the catalytic converter, oxygen sensors and spark plugs. It's impossible to say if all the stuff they changed was 100% necessary, but given that it seems to have fixed the problem at least for a couple of years, it was the right call on their part to ensure the problem was fixed.

Some of the other causes described on the Volvo site include large vacuum leaks and exhaust leaks before the catalytic converter. I would personally start looking for any kind of exhaust leak starting around the exhaust manifold and working my way back to the cat. On one of my Volvos I found a cracked exhaust header, which could definitely cause the problem, but I had to remove the header to find it. Vacuum leaks could be another cause, but one bad enough to cause a P0420 error should also cause noticeable performance problems.

Another guy stated that his P0420 problems went away when he switched to premium gas, and he tested his theory by filling up with regular and sure enough the code came back. Guess it couldn't hurt to fill up with premium, clear the code and see if it comes back.

So hey, go ahead and replace the gas cap, fill 'er up with premium and see what happens!

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@dadibrokeit thank you for your reply. Sorry, I forgot to include the mileage since we replaced the CAT in 03/21. It's been 35,584 miles since this P0420 came back up two weeks ago. Also, if it helps according to my notes, my mechanic used a Bosal brand CAT that has now changed names to BRExhaust. Our mechanic told me that it is still considered OEM grade though and that it's almost exactly Magnaflow minus the sticker. They couldn't do Magnaflow for $400 more because they said it was a different bolt pattern for our car.

In the past couple of years, we have also noticed lack of acceleration in building MPH when accelerating up hills, merging on to the interstate, etc. but we've brought it up a few times to our mechanic and they can't seem to figure out a problem that is causing that so we are left with no results. But not sure if that would have anything to do with this.

If it's not those other issues you mentioned, my wife has said that at times when she fills up gas, the cap seems loose when she is almost positive she tightened it pretty good when last getting gas. We also always get mid-grade gas after replacing the CAT two years ago.

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bd3616 さん、ありがとうございました!
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