Thanks for everyone's posts. Just wanted to give another update since my last post on 12/3. But before I do, when a mechanic changes out a catalytic converter, are the O2 sensors just part of the replacement CAT piece? Or are they a separate piece but they just change them out at the same time because they are in the same area and easy to do? Never understood this if someone could explain, thanks....sorry if I missed someone answering this for me earlier!

Ok so for the update. I mentioned the light came on with that P0420 code on 12/2 (by the way, when I read it on the ODB reader, it comes up as 2 codes but both show as P0420...not sure if that helps or means anything). The check engine light turned off on it's own two days later on 12/4 at 180,310 miles. For the first time, I decided to try premium gas which I got the next day on 12/5 after the light already went off to see if it would help the issue later on. I also got my oil change on 12/11...the light was still off but they hooked it up anyways and no code was stored which they mentioned is a good thing because codes can still be stored even with the light off. I got premium gas a second time on 12/15 and then a third time on 12/21. But then, the check engine light came back on this past Friday on 1/8 at 181,766 miles. I took it to Autozone the next day on 1/9 to make sure it was the same code and sure enough it showed 2 codes like always, both being P0420. I learned from using their OBD reader that I could "erase" the code myself so I did that so currently it is no longer on the dashboard but again who knows when it will come back on again. I also want to make another note that the night before the light came back on 1/8, it was rainy/cold and that morning the car was wet, etc. which seems to be a common occurrence like I've mentioned before. I got premium gas again a fourth time today on 1/10 but the premium gas doesn't seem to be fixing it.

So no idea what to do going forward. Again, I'm not a car guy so like some of the suggestions mentioned, I can't just try replacing the O2 sensor on my own first before replacing the whole CAT like my mechanic mentioned. Again, they mentioned they would look at everything first but seemed to imply they would just replace the CAT all together because they mentioned if they changed out the O2 sensor first and that wasn't it, it would just cost more in the end. It just seems ironic that the light comes on a lot of times when the weather conditions are wet, etc. Just want this problem to be done with once and for all, but just not sure how to move forward lol.

I called my mechanic (who has amazing reviews and has always been trust worthy) and they can't get me in for another week. They again mentioned they would probably just need to replace the CAT all together. They said it probably wouldn't just be the O2 sensors because they said if it was the O2 sensors, it would have brought up another code besides just PO420. There's a chance that when they replace the CAT, they might also need to replace the O2 sensors too though.

 

A shop always likes to replace everything that could possibly be causing the symptom, so you won't be upset if it comes back. Of course, that's the most expensive way to approach it, but it's best for the shop (not for you).

A bad cat can cause that code, or it can be a few other things, including the sensor(s) or even just build-up from oil blow-by or cylinder(s) misfire(s) causing build-up on the cat. In that case, sometimes a road trip will clear up the problem, by subjecting the internals of the cat to more heat and pressure, "cleaning" the working surface. The fact that the code is coming and going would tend to indicate it's a borderline problem - I'd personally tend to swap out the sensor first, since it's relatively cheap and easy. Worst case, it doesn't fix the problem, but you've done a preventative maintenance on one more part that does tend to wear out.

 

A real mechanic (not me) would test the O2 sensor voltage to determine if it's operating properly. The after-cat O2 holds somewhat steady voltage around .5v to indicate the cat converter is doing it's job. Your mechanic doesn't need to guess. There is a test.

Also, a real mechanic with the proper test equipment could also test the exhaust emissions to see if they are out of compliance.

Of course, diagnostic costs are added to the replacement parts costs.

Or you can fire the parts cannon at it and hope for the best. Always been my way. I'm not a mechanic though.

 

Thank you guys for the responses and working through this with me. When a mechanic changes out a catalytic converter, are the O2 sensors just part of the replacement CAT piece? Or are they a separate piece but they just change them out at the same time because they are in the same area and easy to do? Never understood this if someone could explain, thanks....sorry if I missed someone answering this for me earlier!

Again, this check engine light code P0420 first came up in December 2018 for the first time and never came back until July 2020. So in June 2019 I passed emissions with no problems (didn't have to do emissions in June 2020 due to COVID). Does that tell us anything or no? Also, it's not unusual that a CAT would need replacing during the lifespan of a car, correct? Just want to make sure I'm not getting the wool pulled over my eyes from that aspect if this isn't typical and out of the ordinary.

It's just tough because I can't tell my local mechanic what to do in regards to "testing the O2 sensors" out first...everytime I bring it up they have some type of reason for not doing that such as they can't do that or if it was O2 sensors they would bring about another code. So they seem pretty set on just replacing the CAT all together. It's frustrating because they are trust worthy but I just can't get through to them on this. I've called a Volvo independent shop in another close by city and they brought up what you guys mentioned that it could be just O2 sensors which is what they can try replacing first before replacing the whole CAT...but I don't know them as well or how trust worthy they are. Just so confused on who to listen to or what to try to end this problem once and for all! Again, I've never noticed any type of egg smell if that's a for sure symptom of a CAT needing to be replaced though?

Sidenote, my wife was leaving work three days ago on 1/13. She got in the car, turned the key and it got stuck plus the car wouldn't start up. She managed to get the key out but she was noticing the car wouldn't respond by locking/unlocking and tried again to start it back up but nothing was turning on. Sure enough, we got AAA to come out and they tested the battery which was dead. It was purchased back in May 2017. We replaced it with AAA for a new battery. Surely this wouldn't have anything to do with the check engine light/code P0420 after all these years right? I'm sure it doesn't because the first time this issue came up was in December 2018 and never came back until July 2020, just thought it was worth mentioning.

Also, if I replaced the CAT for say $800-$1000...is it still worth it in terms of how much life the car has left? Again it's a 2008 Volvo S40 with 182,000 miles roughly on it. How many miles could I expect to get out of it? Really not wanting to move on from the car yet if I don't have to but also don't want to waste money. But even if we didn't want to change out the CAT and move on from the car in the future...would we still have to replace the CAT first before we got rid of it? I thought I saw someone post on here that it's considered fraud to try and sell or trade a car in without addressing all the issues first?

 

First, if there's any chance that the problem was caused by your bad battery (low voltage), I'd try clearing the code and see if it comes back (you might get lucky, and this costs nothing and takes almost no time).

Second, read my previous post in this thread about the options. If it was my car, I'd start with the cheapest (cat cleaner and some spirited road miles), followed by a new post-cat O2 sensor, followed by the cat (if necessary).

And not to worry - these cars can easily last 300-400,000 miles with some care (not all of it super-cheap, but it's still a very economical car to maintain, IMHO).

 

Don't get too worked up over a P0420. Personally I used the CEL Boss from IPD. There was no way I was going to buy a new cat for an old 2005 car.

Here is a quick YouTube link to the best diagnostic YouTuber I've ever watched. He demonstrates first thing how to check if both O2 sensors are working correctly.

A new cat will not come with O2 sensors. Realize O2 sensors are wear items. Like spark plugs, they need to be replaced every 100k miles.

Test first, then replace as necessary. It's your call whether 800-1000 dollars is worth it to you. To me it is not. An oxygen sensor spacer is the cat's meow.
https://www.ipdusa.com/products/8656...cer-ipd-109553

 

Thanks guys, could the bad battery that I had to replace last week also be anything to do with why the Check Engine Light/P0420 code has been coming up the past few years or probably not? Just seeing if I should still lean towards replacing the CAT or if a bad battery had a chance of causing all this to begin with but I doubt it. The code hasn't been on in 9 days since I cleared it at Autozone myself so I'm guessing that means there is nothing to clear.

 

Anything is possible with a nearly dead battery. If you cleared the code, and it didn't come back, I (personally) wouldn't worry about it. I would check your readiness indicators to make sure that the car will pass inspection though (states will usually allow 1-2 readiness indicators showing "not ready", and still pass the vehicle). If your state doesn't have emissions testing, then yeah - stick a spacer under the rear sensor if it does come back and REALLY forget about it. ;-)

 

Yes but again, the code has been coming on and off for for over 2 years now so just because I clear it doesn't mean it won't come back. It hasn't been on since 9 days ago when I cleared it myself. Also, I called my mechanic and they said the bad battery problem probably wouldn't be the cause of the P0420 code. So frustrating on who to believe with different answers not corresponding.

They also told me if I go with changing out the CAT, my options were:
1. Genuine Volvo for $1496
2. Magnaflow (which is considered OEM grade. But they said it was confusing because on one page it didn't say OEM and another it did) for $1250
3. Bosal for $860

Again I don't know anything about cars so would you guys recommend one of the three the most? They said they would have just done Magnaflow but just wanting to make sure. Also I'm confused because I thought aftermarket and OEM were two different things but aren't Magnaflow and Bosal aftermarket? Yet Magnaflow is OEM grade?

 

Your mech may be right about the P0420 code not being created by a low battery (many, many other codes can be though). I can't say for sure...

"OEM quality" is kind of a nebulous concept - but generally it means that it's a very "legitimate" part (and Magnaflow qualifies for that). I would probably go Magnaflow before the cheaper Bosal or the more expensive Volvo part... Magnaflow has a very good reputation as a high-quality manufacturer.

But - bottom line - if the CEL stays off, drive the car and fuhgeddaboudit. ;-) I'd also consider pouring a bottle of "cat cleaner" in the gas tank next time I was going to be doing some highway miles. If the problem is any kind of build-up or contamination inside the cat, that could well clean it enough to keep it "under the failure threshold" for a long time.

 

Thanks for the reply habbyguy! So is OEM Grade almost as good as Genuine Volvo or no and it's more similar to a true aftermarket product? Just confused where it falls in line in regards to OEM or aftermarket product. What is the difference between OEM Grade Magnaflow and just Magnaflow? And if the Magnaflow isn't OEM grade though...would you still go for it? Or only if it's guaranteed OEM grade? And you don't forsee any issues of replacing the CAT with something different than Genuine Volvo such as this Magnaflow and the CEL coming back on due to it being aftermarket? I've heard that be the case for others...

Yeah, unfortunately the CEL has just continuously come on after trying so many different things. I would love to keep waiting to see if this new battery solves it but I highly doubt it....From my understanding, using premium gas is very similar to "cat cleaner" so I did that numerous times and it still came back on. I'm not very experienced with cars so don't really feel comfortable putting some "additive" in my car...just not experienced enough I'm sorry.

 

On a (well) used Volvo, I'd be happy with anything with Magnaflow stamped on it. Even if it didn't last forever, it's almost certainly going to outlast the vehicle (and I suspect that it's probably at least as good as the OEM cat, even if it doesn't meet every spec).

 

So just an update....by the way does this model only have 1 CAT? Because my mechanic hasn't said anything about more than 1. Again looking at my notes from the printout code it gave me at Autozone all these times said "Catalyst efficiency low-bank 1 Explanation The oxygen sensors monitor the Catalytic Converters ability to store oxygen" and goes on to list probably causes. Not sure if that helps.

I took the car into my mechanic this past Tuesday on 1/19. I told them a week ago that the light came back on 1/8 but hasn't been on since 1/9 because I cleared it myself at Autozone. They had the car all day and called me saying that the good news is the light isn't on...but I'm frustrated because I knew it wasn't on and told them that. They must have forgotten. They said without the light on they couldn't really tell much but even with the light on...they can't really test the CAT out to see if it needs replacing. They said if it were them they wouldn't put the money down right now to replace the CAT because they also don't want it coming back on them if that doesn't solve the issue. They said it shouldn't be a problem driving especially if emissions isn't due and if I'm not noticing any problems. Again...no I'm not noticing the smell but I feel like I am noticing a lack of quicker acceleration, especially going up hills which I said to them. They said they could change out the CAT if I wanted to but you can tell they just don't seem confident. They are very nice though because they were just saying they felt bad me putting that much money down so you can tell they aren't just trying to take my money.

They also said my situation with how the light comes on and off the past few years seems odd because usually they just clear the code for people and it stays off or comes back on and stays on. I had to reclarify that the issue came up for the first time in December 2018 and didn't come again until July 2020 but more often since then which just seemed abnormal to them. But from my research, the P0420/CAT code acts just like my situation for others so that makes no sense to me. They also said another idea would be to take it into the Volvo dealership for their confirmation and then if they confirm it they could do it for me. But they also said the Volvo dealership wouldn't be able to tell any additional information either without the light on....so now I feel stuck. I can't predict when this light will go on and off and even so if you can't tell for sure the CAT needs replacing when the light is on like they said...how will this help. So now I'm super frustrated because I have no idea how to proceed especially if they aren't confident anymore when they were all along until now. Every other time the past 1-2 years when I tell them the code I'm getting back, they said it's probably the CAT needing to be replaced. Plus when I took the car into them back in August with the light on, they just cleared it that day. So they had the opportunity to check it out then with the light on. They told me on this past Tuesday if the light comes on again, it might make them feel more confident which makes no sense to me because it has constantly been coming on and off the past few years and I can't guarantee bringing it into them with the light on.

I understand a lot of you will say don't worry about it and to not fix anything. Usually I would believe you but the fact this keeps coming back makes me believe something needs fixing. Plus it's been 2 years now which worries me more. No idea how to move forward now. I obviously don't want them replacing it based off just my opinion because I don't know anything about cars...I want someone confident to say that's what it is and to move on from this. So fed up with this.

 

Keep in mind that the error code indicates ONLY that - worst case - your car may be putting out incrementally more CO2 than it's designed to. That won't affect power, mileage, or reliability. Just a little more CO2 coming out your exhaust pipe, and an annoying light on your dash.

Listen to the mechs - they're right. If the light's not on, the system is working to spec, and there's really nothing to gain by swapping out what may be a perfectly good catalytic converter. And yes, even if they did, and the problem was really something else (like a flakey O2 sensor or wiring fault), your CEL would come back on just as quickly as it would have with the old cat.

If this was my mother's car, I'd tell her to drive it and not worry a microsecond about it. I know the bozos selling extended warranties make a CEL sound like ebola, but in this case, it's really just a minor annoyance.

Oh, and yes - only one cat. With a five cylinder, no one can do the math to figure out how to split the cylinders across multiple cats. ;-)

 

Thanks for the reply, habbyguy. I understand your point but if the light keeps coming back...isn't that indicative that something is wrong? I've heard people have bad CATS even with the CEL coming on and off...not just staying on all the time, no? I'm just curious how we would know that the CAT does need to be fixed and aren't these are all signs, no? I mean, I have noticed what I believe is maybe a little less quicker power/acceleration putting my gas on the pedal when merging on interstate, etc and from what I hear this is one symptom.

So just another update. Check engine light came back on 2/12/21 @ 183,296 miles when it was roughly 28 degrees out that morning, took it to Autozone and again confirmed code P0420 like always. We had a snow storm in our area a few days later so the car wasn't driven for a week and once we started driving it again on 2/22/21....the check engine light went off on 2/24/21 @ 183,544 miles when it was roughly 65 degrees out. I'm kinda frustrated because when the check engine light came back on 2/12, I called a second mechanic and set up an appointment on 3/2 to bring it in just to see if they could determine anything with the issue. They told me that they would prefer the check engine light being on in order to help diagnosis it better....but of course the light went off on 2/24 so now I have no idea whether I should bring it in or not because it could be a waste of time with no light on. I also was doing nothing but premium gas for almost 6 fill ups hoping this would fix the issue but the check engine light came back on 2/12 so I'm done trying that out because it obviously didn't work. This second mechanic said they do use VIDA while my first mechanic said they do not and only use Autologic and Autel which has most Volvo things but not all. If you recall, one time I took the car in to my first mechanic with the light off and they said no codes were stored which is a good thing because codes can still be stored even with the check engine light off.

So just another frustrating update and not sure how to proceed. I understand that I might should just not worry about it but I'm also not a risk taker so something worsening scares me too. Hope these updates help someone.

 

There's nothing we're going to say that's going to stop you from worrying about that P0420 code stopping your car in the middle of the Sahara Desert, so I suggest just going to the mechanic with the VIDA equipment, and giving him your checkbook. ;-)