# DO I NEED TO REPLACE MY CATALYTIC CONVERTER P0420



## liotb1701 (Jul 28, 2021)

Good day, and thank you ahead of time for your time and effort!

THE PATIENT: Altima, S, 2006, 2.5L
THE PROBLEM: The car sometimes has a stuttering start-up. Check engine light on. P0420 CODE. Temp after CAT is significantly lower than the temp before the CAT.
WHAT HAS BEEN DONE: I replaced spark plugs and changed the position of coil packs. Nothing changed. I didn't change the O2 sensors as I wanted to ask here before I did that.
THE QUESTION: Is this problem all in the CAT.?
FOLLOW UP: If it is, what are some tips for DIY removal, installation, and making sure I have the right one (fit, Utah emissions, etc.) with decent quality for a 200k mil car? How big of a deal is it if I don't get new O2 sensors at this time if I do need to replace the CAT.?

Again, thank you ahead of time!

I have attached some OBD scans that I did that might help.


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## VStar650CL (Nov 12, 2020)

Yep, most likely that's a bad cat. If you look at your second attachment, the swing time on the sensor is healthy which means it probably isn't a bad sensor. If the car isn't down on power then the cat probably hasn't melted much, but it isn't doing its job. You don't want to let it melt, Nissan engines are all Miller-cycle to avoid the need for an EGR, but that also means the engine will inhale a cloud of molten metal if the cat disintegrates. That's bye-bye motor 100% of the time, the cylinder scoring is instantaneous and horrendous.


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## liotb1701 (Jul 28, 2021)

VStar650CL said:


> Yep, most likely that's a bad cat. If you look at your second attachment, the swing time on the sensor is healthy which means it probably isn't a bad sensor. If the car isn't down on power then the cat probably hasn't melted much, but it isn't doing its job. You don't want to let it melt, Nissan engines are all Miller-cycle to avoid the need for an EGR, but that also means the engine will inhale a cloud of molten metal if the cat disintegrates. That's bye-bye motor 100% of the time, the cylinder scoring is instantaneous and horrendous.


Hello VStar650cl,
I really appreciate that insight! Yikes, I need to get this puppy fixed soon then. Do you have any suggestions on the replacement process and parts? I was just going to do it on my own and often an heads up from somebody is really helpful.

Again, thank you!


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## VStar650CL (Nov 12, 2020)

liotb1701 said:


> I really appreciate that insight! Yikes, I need to get this puppy fixed soon then. Do you have any suggestions on the replacement process and parts? I was just going to do it on my own and often an heads up from somebody is really helpful.
> 
> Again, thank you!


You're most welcome! For parts, aftermarket is okay but don't go cheap-cheap. Rock Auto carries Eastern Catalytic, which my customers have done well with, and @smj999smj and I both agree on Bosal. I've seen cheap China stuff fail inside of 10K miles and make for some very unhappy people, so don't do it.

Be prepared with some new heat shield screws (they always snap), and some fresh head-studs and nuts as well (it isn't uncommon for the studs to come out of the head instead of the nuts spinning off). Remove the front heat shield beforehand so you can soak the fasteners for at least a day, and the same for the flex-pipe studs underneath and the A/F (front O2) sensor. The longer you can soak them the better. If the flex-pipe studs spin out with the nut it's okay to re-use them as if they're a bolt, but don't do that with the head studs, it will mess up your torque when you re-install. The A/F's are often stubborn and the best way to get it moving is to take it out with the cat, lock it in a vise, then use a slotted sensor socket with a band clamp around the bottom to keep it from spreading. It's okay to tap-tap it gently with a hand-sledge if necessary, a bit of impact will often serve you better than a lot of torque. Lastly, don't put it back together without the front heat shield. Most people don't realize that glycol vapor has the lowest flash point of any substance in the car, and that shield is there to prevent a calamity if your radiator ever springs a leak.


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## 2016AltimaS (Jul 28, 2021)

liotb1701 said:


> Good day, and thank you ahead of time for your time and effort!
> 
> THE PATIENT: Altima, S, 2006, 2.5L
> THE PROBLEM: The car sometimes has a stuttering start-up. Check engine light on. P0420 CODE. Temp after CAT is significantly lower than the temp before the CAT.
> ...


I have been going through this. If you had a bad O2 sensor, you would get a different code but O2 sensors could cause the po420 code also. I also have a blue driver but, I have been dealing with the po420 code for the last year, I have 232K on my 2016 2.5 L Altima if I were you and I will tell you what I did to remedy this way before I changed the CAT, Get some Cataclean and follow the directions. This worked for me twice in the last year. Eventually you will need to replace the CAT. Be sure to check out Rockauto for the part, they have many different ones. Also, I have gone through 6 upstream O2 sensors because I kept trying to save money and but the NTK, Bosch etc But, BUY the DENSO get it at NAPA it has the correct length or wires to connector. Using the others even the Bosch which was $250 i got codes such as the sensor was taking to long to respond. Denso is the best sensor.


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## VStar650CL (Nov 12, 2020)

2016AltimaS said:


> I have 232K on my 2016 2.5 L Altima if I were you and I will tell you what I did to remedy this way before I changed the CAT, Get some Cataclean and follow the directions. This worked for me twice in the last year. Eventually you will need to replace the CAT.


I've dealt with spurious P0420's on my own 160K '04 due to the previous owner installing a cheap Walker cat. Its efficiency really is low and the sensor knows it. For the moment I ended up resorting to a sensor spacer, but that will change when I see a good deal on a better cat. 

CataClean may well work for awhile, that's also true. Or it may mask a cat that's about to come apart and destroy the engine. You pay your two bits and take your chances with those sorts of solutions.


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## 2016AltimaS (Jul 28, 2021)

VStar650CL said:


> I've dealt with spurious P0420's on my own 160K '04 due to the previous owner installing a cheap Walker cat. Its efficiency really is low and the sensor knows it. For the moment I ended up resorting to a sensor spacer, but that will change when I see a good deal on a better cat.
> 
> CataClean may well work for awhile, that's also true. Or it may mask a cat that's about to come apart and destroy the engine. You pay your two bits and take your chances with those sorts of solutions.


The Walker Cat I just put in, cost $500 it was one of the more expensive cats on Rockauto. Why do you call it cheap? I should have purchased the cheaper one on Rockauto . What do you know about a Walker cat?


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## 2016AltimaS (Jul 28, 2021)

VStar650CL said:


> I've dealt with spurious P0420's on my own 160K '04 due to the previous owner installing a cheap Walker cat. Its efficiency really is low and the sensor knows it. For the moment I ended up resorting to a sensor spacer, but that will change when I see a good deal on a better cat.
> 
> CataClean may well work for awhile, that's also true. Or it may mask a cat that's about to come apart and destroy the engine. You pay your two bits and take your chances with those sorts of solutions.


What would have been the better cat to use? Dealer charges freaking $900 for the cat . If not for having to remove the alternator belt I would have replaced it myself. Putting the belt back on the alternator is a huge pain in the ass


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## VStar650CL (Nov 12, 2020)

Some Walker stuff is great, some is cheap. I didn't mean to malign the brand in general, but a good deal of it is China-cheap and you need to watch what you're getting. Dealer prices are indeed too high, but not as high as replacing it twice.


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## 2016AltimaS (Jul 28, 2021)

VStar650CL said:


> Some Walker stuff is great, some is cheap. I didn't mean to malign the brand in general, but a good deal of it is China-cheap and you need to watch what you're getting. Dealer prices are indeed too high, but not as high as replacing it twice.


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## 2016AltimaS (Jul 28, 2021)

2016AltimaS said:


> View attachment 7616


Is this one crap. Lord I hope not


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## 2016AltimaS (Jul 28, 2021)

2016AltimaS said:


> The Walker Cat I just put in, cost $500 it was one of the more expensive cats on Rockauto. Why do you call it cheap? I should have purchased the cheaper one on Rockauto . What do you know about a Walker cat?


The cat said it was manufactured in the USA


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## 2016AltimaS (Jul 28, 2021)

VStar650CL said:


> I've dealt with spurious P0420's on my own 160K '04 due to the previous owner installing a cheap Walker cat. Its efficiency really is low and the sensor knows it. For the moment I ended up resorting to a sensor spacer, but that will change when I see a good deal on a better cat.
> 
> CataClean may well work for awhile, that's also true. Or it may mask a cat that's about to come apart and destroy the engine. You pay your two bits and take your chances with those sorts of solutions.


Ok I am not being sarcastic but, I would really know how a bad cat could destroy an engine. Please explain? I mean heck, people delete cats the purpose of the cat is to decrease emissions???


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## VStar650CL (Nov 12, 2020)

2016AltimaS said:


> Is this one crap. Lord I hope not


You tend to get what you pay for with cats, platinum and palladium ain't cheap and neither are the high temperature alloys used in making good honeycomb. For what you paid for it, and given that it was made here, I'm sure that one is fine.



2016AltimaS said:


> Ok I am not being sarcastic but, I would really know how a bad cat could destroy an engine. Please explain? I mean heck, people delete cats the purpose of the cat is to decrease emissions???


Cats often self-destruct when they wear out, the honeycomb melts and goes to pieces. Nissan engines all use Miller-cycling to eliminate the need for an EGR, which is a technique of manipulating valve overlap to cause some exhaust gas to flow back through the exhaust valve into the cylinder. That reverse inhalation is what makes a bad cat dangerous. If it comes apart the engine will suck in the debris, and the result is catastrophic 100% of the time.


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## 2016AltimaS (Jul 28, 2021)

VStar650CL said:


> You tend to get what you pay for with cats, platinum and palladium ain't cheap and neither are the high temperature alloys used in making good honeycomb. For what you paid for it, and given that it was made here, I'm sure that one is fine.
> 
> 
> 
> Cats often self-destruct when they wear out, the honeycomb melts and goes to pieces. Nissan engines all use Miller-cycling to eliminate the need for an EGR, which is a technique of manipulating valve overlap to cause some exhaust gas to flow back through the exhaust valve into the cylinder. That reverse inhalation is what makes a bad cat dangerous. If it comes apart the engine will suck in the debris, and the result is catastrophic 100% of the time.


 oK, THAT MAKES SENSE. thanks


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## 2016AltimaS (Jul 28, 2021)

VStar650CL said:


> You tend to get what you pay for with cats, platinum and palladium ain't cheap and neither are the high temperature alloys used in making good honeycomb. For what you paid for it, and given that it was made here, I'm sure that one is fine.
> 
> 
> 
> Cats often self-destruct when they wear out, the honeycomb melts and goes to pieces. Nissan engines all use Miller-cycling to eliminate the need for an EGR, which is a technique of manipulating valve overlap to cause some exhaust gas to flow back through the exhaust valve into the cylinder. That reverse inhalation is what makes a bad cat dangerous. If it comes apart the engine will suck in the debris, and the result is catastrophic 100% of the time.


Where did you get the spacer? do you know the part number? What are signs of the cat having low efficiency?


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## VStar650CL (Nov 12, 2020)

2016AltimaS said:


> Where did you get the spacer? do you know the part number? What are signs of the cat having low efficiency?


The spacers are all over eBay and Amazon, search "O2 sensor extender" and you should get plenty of hits. By moving the sensor back out of the flow an inch or two you fool it into thinking all is well. It doesn't stop it from working, but it makes it more tolerant of low efficiency. The sensor doesn't only measure voltage, it also swings over time in response to cyclic leaning and riching from the ECM. Slower or faster than normal swings will generally be interpreted by the ECM as a bad sensor, whereas abnormal voltage swings with normal response time will be interpreted as a weak cat.


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## smj999smj (Jan 1, 2006)

2016AltimaS said:


> Ok I am not being sarcastic but, I would really know how a bad cat could destroy an engine. Please explain? I mean heck, people delete cats the purpose of the cat is to decrease emissions???


A lot of today's vehicles have eliminated "traditional" EGR systems and are using the variable camshaft timing to accomplish exhaust gas recirculation. It works by having the exhaust valve open slightly during an intake stroke of the piston and pulling the inert exhaust gas back past the exhaust valve and into the combustion chamber. Sounds good on paper! Nissan started using this on the QR25DE engine on the 2002 Altima (and some Sentras). On this engine, the upstream converter is built into the exhaust manifold. Something in the ECM programming caused the catalyst substrate to come apart and bits of this substrate got sucked into the combustion chambers during this EGR process. This catalyst debris would cause scratching of the cylinder bore wall surface and ruin the seal between the cylinder wall and the piston rings, resulting in low compression and oil burning engines. Nissan later addressed the issue with ECM reprogramming, and some added heat shields during a recall. Those that showed damaged converters would get a new exhaust manifold/converter and those engines that failed a compression test would get an engine replacement. 

If one uses a poor quality catalytic converter and it's substrate breaks apart, it could result in damaging the engine in a similar way. 

As far as Walker converters and exhaust, I've had good luck with them. I have a Walker Ultra converter on my 06 Pathfinder that's given me no problems for the seven or eight years it's been installed. I put Walker in the same category as Bosal, although some of the Bosal exhaust parts seem a little better made, IMO. I have no issues with most of the brands of converters that Rockauto sells, although I did have an issue with a Davison Y-pipe/catalytic converter assembly on a Chevy truck that continued to through catalyst codes after install (it was replaced with a Bosal part and the problem was fixed). The ones you really have to watch out for are the really inexpensive converters you see on sites like Ebay, almost always made in China. One also encounters a lot of counterfeit auto parts on Ebay, so buyer beware! If the price is too good to be true, it probably is.


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## 2016AltimaS (Jul 28, 2021)

smj999smj said:


> A lot of today's vehicles have eliminated "traditional" EGR systems and are using the variable camshaft timing to accomplish exhaust gas recirculation. It works by having the exhaust valve open slightly during an intake stroke of the piston and pulling the inert exhaust gas back past the exhaust valve and into the combustion chamber. Sounds good on paper! Nissan started using this on the QR25DE engine on the 2002 Altima (and some Sentras). On this engine, the upstream converter is built into the exhaust manifold. Something in the ECM programming caused the catalyst substrate to come apart and bits of this substrate got sucked into the combustion chambers during this EGR process. This catalyst debris would cause scratching of the cylinder bore wall surface and ruin the seal between the cylinder wall and the piston rings, resulting in low compression and oil burning engines. Nissan later addressed the issue with ECM reprogramming, and some added heat shields during a recall. Those that showed damaged converters would get a new exhaust manifold/converter and those engines that failed a compression test would get an engine replacement.
> 
> If one uses a poor quality catalytic converter and it's substrate breaks apart, it could result in damaging the engine in a similar way.
> 
> As far as Walker converters and exhaust, I've had good luck with them. I have a Walker Ultra converter on my 06 Pathfinder that's given me no problems for the seven or eight years it's been installed. I put Walker in the same category as Bosal, although some of the Bosal exhaust parts seem a little better made, IMO. I have no issues with most of the brands of converters that Rockauto sells, although I did have an issue with a Davison Y-pipe/catalytic converter assembly on a Chevy truck that continued to through catalyst codes after install (it was replaced with a Bosal part and the problem was fixed). The ones you really have to watch out for are the really inexpensive converters you see on sites like Ebay, almost always made in China. One also encounters a lot of counterfeit auto parts on Ebay, so buyer beware! If the price is too good to be true, it probably is.


Thank you for all of that useful information. Good to hear that you have had good luck with the Walkers. Because today when I got my car back after only installed the Walker yesterday, it could be my imagination but, it seems to not have the pick up that it had even the day before with the still bad converter on there. And then when I got here and saw that the other poster was saying that they were not that good, I started thinking I made a wrong choice with the one that I got. When I was looking at the different ones on Rockauto, there were several options anywhere from as low as $148, I think up to the factory one that they sold and the one I got was I believe the third from the top of the list as far as price. And it looked just like mine. I know about the precious metals but did not think that they would scrimp on them and maybe that is why I may have purchased the dealer one. But, I have so many miles on my car and soon the tranny will have to be replaced for the second time. I hope I was just imaging things when I drove it today.


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## smj999smj (Jan 1, 2006)

2016AltimaS said:


> Thank you for all of that useful information. Good to hear that you have had good luck with the Walkers. Because today when I got my car back after only installed the Walker yesterday, it could be my imagination but, it seems to not have the pick up that it had even the day before with the still bad converter on there. And then when I got here and saw that the other poster was saying that they were not that good, I started thinking I made a wrong choice with the one that I got. When I was looking at the different ones on Rockauto, there were several options anywhere from as low as $148, I think up to the factory one that they sold and the one I got was I believe the third from the top of the list as far as price. And it looked just like mine. I know about the precious metals but did not think that they would scrimp on them and maybe that is why I may have purchased the dealer one. But, I have so many miles on my car and soon the tranny will have to be replaced for the second time. I hope I was just imaging things when I drove it today.


The lack of pickup may have nothing to do with the catalytic converter. Usually when they trigger a P0420 or P0430 catalyst code, it's because the ECM has determined that the catalyst is not breaking down the exhaust emissions efficiently, rather than because it is clogged or restrictive. Of course, and exhaust back pressure tester can determine if the exhaust system, including the converters, is restricted. Since you say the tranny needs to be replaced, perhaps that's the reason for the vehicle's lack of pickup?


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## liotb1701 (Jul 28, 2021)

VStar650CL said:


> You're most welcome! For parts, aftermarket is okay but don't go cheap-cheap. Rock Auto carries Eastern Catalytic, which my customers have done well with, and @smj999smj and I both agree on Bosal. I've seen cheap China stuff fail inside of 10K miles and make for some very unhappy people, so don't do it.
> 
> Be prepared with some new heat shield screws (they always snap), and some fresh head-studs and nuts as well (it isn't uncommon for the studs to come out of the head instead of the nuts spinning off). Remove the front heat shield beforehand so you can soak the fasteners for at least a day, and the same for the flex-pipe studs underneath and the A/F (front O2) sensor. The longer you can soak them the better. If the flex-pipe studs spin out with the nut it's okay to re-use them as if they're a bolt, but don't do that with the head studs, it will mess up your torque when you re-install. The A/F's are often stubborn and the best way to get it moving is to take it out with the cat, lock it in a vise, then use a slotted sensor socket with a band clamp around the bottom to keep it from spreading. It's okay to tap-tap it gently with a hand-sledge if necessary, a bit of impact will often serve you better than a lot of torque. Lastly, don't put it back together without the front heat shield. Most people don't realize that glycol vapor has the lowest flash point of any substance in the car, and that shield is there to prevent a calamity if your radiator ever springs a leak.


Thank you, VStar650CL,
for the perfectly articulated information and insight. I was looking at this on RockAuto: More Information for AP / EASTERN 641519. What do you think? Also, what penetrating oil do you recommend? Where are the best places to get new head-studs and nuts? If the studs come out "okay" then do they need to be retapped at all? What's the best place to find the torque specks on the head-studs and nuts?

Thank you for your continued support!


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## VStar650CL (Nov 12, 2020)

I've never had a customer go wrong with an Eastern cat. PB Blaster is the best penetrant in my book. For the studs and nuts, go to NissanPartsDeal.com and plug in your VIN number, they're a consortium of dealers and have the same database and drawings as Nissan DPC. That will give you the Nissan p/n's for your exact ride, and you can use those if you want to search aftermarket. I can get you torque specs, will post them later when I have more time, or maybe Rogo has them in his data collection and can help out..


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## liotb1701 (Jul 28, 2021)

VStar650CL said:


> I've never had a customer go wrong with an Eastern cat. PB Blaster is the best penetrant in my book. For the studs and nuts, go to NissanPartsDeal.com and plug in your VIN number, they're a consortium of dealers and have the same database and drawings as Nissan DPC. That will give you the Nissan p/n's for your exact ride, and you can use those if you want to search aftermarket. I can get you torque specs, will post them later when I have more time, or maybe Rogo has them in his data collection and can help out..


Perfect, I will look those up. Oh, thank you, but you don't need to go through the trouble to look up the specs, if you will point me in the right direction I can do that. Very generous of you!

As I was going through the diagnostics again and found this report. What do you make if it?


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## VStar650CL (Nov 12, 2020)

liotb1701 said:


> Perfect, I will look those up. Oh, thank you, but you don't need to go through the trouble to look up the specs, if you will point me in the right direction I can do that. Very generous of you!


Too late, I already did. Manifold nuts are 29~32 ft/lbs, flex pipe nuts are 38~47. A/F Sensor is 30~36 and Nissan and I both recommend anti-seize on the threads.



liotb1701 said:


> As I was going through the diagnostics again and found this report. What do you make if it?


Hard to tell anything from that plot without seeing what the throttle was doing. But like I said, the ECM's are pretty good at discriminating a dying cat from a dying sensor, and with the mileage on your ride the former is entirely likely.


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## liotb1701 (Jul 28, 2021)

VStar650CL said:


> Too late, I already did. Manifold nuts are 29~32 ft/lbs, flex pipe nuts are 38~47. A/F Sensor is 30~36 and Nissan and I both recommend anti-seize on the threads.
> 
> You are a scholar and a gentleman. Thank you for taking the time to do that. Dumb question, but was hat anti-seize only for the sensor?
> 
> Hard to tell anything from that plot without seeing what the throttle was doing. But like I said, the ECM's are pretty good at discriminating a dying cat from a dying sensor, and with the mileage on your ride the former is entirely likely.


Oh, I feel dumb, that was one of the data sets I attached in the original post. I got it confused with something else. If I remember correctly that was around 2k RPMs.

Again, thank you for all of your help!


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## VStar650CL (Nov 12, 2020)

Yep, anti-seize only for the sensor. It may mess up your torque on the other fasteners. Don't bother with torque on the heat shield screws, just be gentle. They don't need much and will quickly corrode into permanence. If you get carried away and break one it's a mess.


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## liotb1701 (Jul 28, 2021)

VStar650CL said:


> Yep, anti-seize only for the sensor. It may mess up your torque on the other fasteners. Don't bother with torque on the heat shield screws, just be gentle. They don't need much and will quickly corrode into permanence. If you get carried away and break one it's a mess.


Good to know, thank you. You have been most helpful. I will get the CAT ordered, pick a time when I have a few days off. I really appreciate your time and insights.


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## Joseph_David_2312 (4 mo ago)

I was planning to do it on my own, but getting a heads up from someone is frequently quite beneficial.


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