Return of P0420 code?

@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?

@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.

@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.

@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.

@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.