# Altima Service Engine Light- P0420 code



## Altima98 (Dec 9, 2006)

The service engine light on my 98 Altima has turned ON. The code scanner read P0420 and the dealer recommended a new CAT with some accessories for a whopping $1200 US$. The codes were cleared by the computer at the dealer. After driving about 10-15 miles, the MIL is again back. On the flip side, I do not see any problems with the engine, drive ability or idling. The dealer also said it is unlikely that the O2 sensors are bad as they have a separate code which was not seen during the diagnosis. Now,
1.	Can I continue driving the car without going for the expensive fix?
2.	The dealer said it is not recommended to drive the car at speeds above 50mph with the MIL ON. Is this true?
3.	Can somebody explain me how I can reset the MIL by disconnecting the battery? Or any links that explain this?

Thanks


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## Toolman5523 (May 23, 2007)

The cat is there as an emissions control device. You can drive it without fixing it, so long as you don't mind making the sky bleed (I guess you don't care about our children's future ). You can reset the light all you want the light will just keep coming on. As far as drive speeds with a chk engine light, well, you really should fix it as soon as possible.


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## Kraut-n-Rice (Jan 10, 2007)

Toolman5523 said:


> The cat is there as an emissions control device. You can drive it without fixing it, so long as you don't mind making the sky bleed (I guess you don't care about our children's future ). You can reset the light all you want the light will just keep coming on. As far as drive speeds with a chk engine light, well, you really should fix it as soon as possible.



The obvious thing is also that just a catalyst replacement and O2 sensors may not be the root of the problem . If the oxygen sensors were bad or out of range they should have set a code anyway before th cat failure. The cat is probably bad due to a rich mixture from another failed component . Scanning the other sensors ,engine temp , MAF , air temp sensor ,etc to see if they are operating within range should be considered before throwing a wad of money at the car that may not fully repair it.


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## Brianz01Altima (Jan 17, 2007)

The cats are know for going out on these cars at about 120k plus. 

Unfortunately, like the previous guys said there are many factors that can cause this code, such and intake gasket leaks, fuel injector leaks, exhaust system leaks, or faulty O2 sensors.. Unless you have the ability to test the above items it will get expensive replacing parts..

You don't need to change the way you drive the car. I drove mine normally for almost 10k before I diagnosed and replaced the cat.


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## Altima98 (Dec 9, 2006)

Thanks everyone for the responses.


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## Altima98 (Dec 9, 2006)

Disconnected the battery today for 15 min and the MIL is still ON. Looking for further diagnosis...


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## MickeyKnox (Jun 17, 2006)

so you need to do a tune up before you pull codes. it may not be your can converter. maybe it's the rear O2 censor. you can buy both O2 censors at NGK.com. you most probably have front pre cat converter. and it's most probably melted and clogged(this can lead to engine overheating and failure). take it out and beat it out. if you are not in California you don't need front cat converter. front cat converter is right under exhaust manifold. right where exhaust gases leave the engine. 

but removing front pre cat converter is a must for all altima owners.


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## Altima98 (Dec 9, 2006)

MickeyKnox,
Please explain, " removing front pre cat converter is a must for all altima owners" from your previous reply.









MickeyKnox said:


> so you need to do a tune up before you pull codes. it may not be your can converter. maybe it's the rear O2 censor. you can buy both O2 censors at NGK.com. you most probably have front pre cat converter. and it's most probably melted and clogged(this can lead to engine overheating and failure). take it out and beat it out. if you are not in California you don't need front cat converter. front cat converter is right under exhaust manifold. right where exhaust gases leave the engine.
> 
> but removing front pre cat converter is a must for all altima owners.


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## Brianz01Altima (Jan 17, 2007)

What he means is California models have a front cat that mounts on the exhaust manifold and a second one that mounts downstream. Federal models only have one converter at the exhaust manifold.

Some folks with the federal models have gutted the catalyst from the cat on the exhaust manifold and have a less expensive unit installed downstream. A good exhaust shop can help with this.


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## MickeyKnox (Jun 17, 2006)

Altima98 said:


> MickeyKnox,
> Please explain, " removing front pre cat converter is a must for all altima owners" from your previous reply.


bryan is right about cat converters locations. some altimas have only one catalytic converter which is downstream. underneath the middle of your car. pre cat is located inside the header or exhaust manifold. to check if you have it you need to look underneath the heat shield of your exhaust manifold.(where the front Oxygen censor is). if you have something like this:








then you have front catalytic converter. I forgot the years. 95-97 altimas have precats. 93-94 don't. I think.
the problem with pre cat is that it is located right where exhaust gases are the hottest. so it melts. downstream the exhaust pipe gases cool down and are not as hot. and rear catalytic converter is intact most of the time. clogged precat --> exhaust gases are trapped--> huge back pressure against pistons--> pistons have to do additional work(push regular combustion gases and gases trapped in exhaust manifold). can lead to overheating of the engine.

a lot of people take out precats and beat them out with a screwdriver and a hummer. mine was about 50-60% melted. it looks ok from outside, but once your start beating it out you see a lot of melted catalyst. you will still pass emissions if you are outside california and get 3-5hp depending on how clogged pre cat is.
if you have money buy obx 4-2-1 header. or if you find get hotshot header.


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## Altima98 (Dec 9, 2006)

What are these "Headers"? and how do they help my situation?






MickeyKnox said:


> bryan is right about cat converters locations. some altimas have only one catalytic converter which is downstream. underneath the middle of your car. pre cat is located inside the header or exhaust manifold. to check if you have it you need to look underneath the heat shield of your exhaust manifold.(where the front Oxygen censor is). if you have something like this:
> 
> 
> 
> ...


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## Altima98 (Dec 9, 2006)

Thanks all for sharing your thoughts.


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## MickeyKnox (Jun 17, 2006)

Altima98 said:


> Thanks all for sharing your thoughts.


in your situation you need to do complete tune up before you pull codes. very often computer gives erroneous codes because there is an air leak, dirty oil, dirty air filter, bad spark plugs which all can cause poor emissions. 

as for headers. it's best that you go to howstuffworks.com and learn about combustion engine , exhaust, catalytic converter and why it may malfunction. it's very helpful. header = exhaust manifold.

then what you read here will make a lot of sense and you will have a better idea what parts exactly you need to change. because changing cat converter may not eliminate the problem. I also had the same code. in my case complete tune up fixed it.


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## Altima98 (Dec 9, 2006)

*Reliable & Economical Catalytic Converter*

I am looking for a reliable & economical catalytic converter for my 98 Altima-2.4L GLE with Federal exhaust manifold. I believe the Universal Fit converters would not work in my case. My options are: OEM CAT ( Very expensive) or After market units (CATCO, D.E.C, Eastern , Walker etc). These are relatively cheaper. My questions:
1. Can I use these after market units reliably on my Altima?
2. Are these really direct fits with OEM spec's as mentioned without the need to do extra cutting, welding etc
3. Which of these brands are recommended?

REF: http://www.carpartswholesale.com/cp...&year=1998&part_name=catalytic_converter&my=1


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## MickeyKnox (Jun 17, 2006)

if you decide to get new cat converter get magnaflow high flow cat converter. call around your automotive stores, exhaust stores. they should have it for around 70-80$. welding is 30-50$ at most. Most shops do cat converter installation for 100$ if you buy cat converter form them.
you will have to weld it to your pipe. you will pass emissions. but change your rear O2 sensor first. it's most probably your rear O2 sensor that is reading your exhaust gases incorrectly. 

there is also spark plug fix of your infamous code. search online. basically spark plug fix gets your rear O2 sensor farther away from cat converter. so your exhaust reading is lower.


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## Altima98 (Dec 9, 2006)

MickeyKnox: I took my car to a local muffler shop and they said they can not cut and weld the universal CATs because of the location of the CAT in the engine manifold area (98 ALTIMA) or atleast something like that. They ruled out the possibility of using an Universal CAT on my car. Also, the dealer ruled out the possibility of a bad oxygen sensor during diagnosis. He said they would have triggered a different code incase the sensors were bad. So I started looking for Direct fit -OEM replacement as they are cheaper compared to OEM. I am not sure which is a better choice:CATCO, D.E.C, Eastern or Walker?


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## MickeyKnox (Jun 17, 2006)

so trust me on this one. you don't need front cat converter(the one in exhaust manifold near the engine). if you don't live in california you can take the exhaust manifold and beat this precat out. you will pass emissions provided your rear cat is working. you have 2 cat converters it seems. on all cars front cat converter is nothing but trouble maker. it melts and is almost always clogged after 100K. when was the last time you changed rear O2 sensor? good idea to change it at around 150K. I had exact same code. I was thinking of buying new cat converter. but then found out I must beat out front cat converter and change rear O2 sensor. I just changed rear O2 sensor and the problem was solved. 3 months ago I beat out the front precat. and it's like getting a second engine in your car.

also codes are not 100% accurate. if your car is not tuned up it will give you all kinds of erroneous codes.


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## Altima98 (Dec 9, 2006)

MickeyKnox said:


> so trust me on this one. you don't need front cat converter(the one in exhaust manifold near the engine). if you don't live in california you can take the exhaust manifold and beat this precat out. you will pass emissions provided your rear cat is working. you have 2 cat converters it seems. on all cars front cat converter is nothing but trouble maker. it melts and is almost always clogged after 100K. when was the last time you changed rear O2 sensor? good idea to change it at around 150K. I had exact same code. I was thinking of buying new cat converter. but then found out I must beat out front cat converter and change rear O2 sensor. I just changed rear O2 sensor and the problem was solved. 3 months ago I beat out the front precat. and it's like getting a second engine in your car.
> 
> also codes are not 100% accurate. if your car is not tuned up it will give you all kinds of erroneous codes.



I bought this car at about 100K and is now about 125K. I have no idea when the oxygen sensors were last changed. Thanks for the wonderful suggestions. I will drop the idea of changing the precat for now and will change the oxygen sensor and see if I can get the precat beaten out by my mechanic. I will update you again. Thanks again...


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## MickeyKnox (Jun 17, 2006)

I strongly advice to do it yourself. it's an easy job. good way to start working on cars.

mechanics charge a lot of money for simple do it yourself jobs.


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## OE812 (Jun 4, 2006)

MickeyKnox:

Explain the steps in gutting of the pre cat for me. I have the dreaded P0420 code on my 2000 Altima.


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## missouriupser (Oct 6, 2019)

I am looking at these post and they are really old so here I go, I have a 2008 Nissan Altima and it had a service engine soon light come on so I checked the gas cap immediately and tighten. Has anyone bought a new gas cap to begin the process of elimination to see if that's the problem? 
I went to AutoZone and they ran a code check and it shows the PO420 code and saying the CC is probably needing replaced. Then I read in above post to change the 02 sensors first. Can anyone who has had this problem get me headed in the right direction?


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## quadraria10 (Jul 6, 2010)

Your Altima is 11 years old-- the old gas cap is typically related to a new car that gets a check engine light. In this thread and others people have told you that it could be a number of things. It could be that the O2 sensor is no longer providing good readings to the ECU, but it could also be that the O2 sensors are providing the correct reading and there is a deeper underlying problem. You need to investigate or pay someone to. Good luck.


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## rogoman (Dec 16, 2004)

missouriupser said:


> I am looking at these post and they are really old so here I go, I have a 2008 Nissan Altima and it had a service engine soon light come on so I checked the gas cap immediately and tighten. Has anyone bought a new gas cap to begin the process of elimination to see if that's the problem?
> I went to AutoZone and they ran a code check and it shows the PO420 code and saying the CC is probably needing replaced. Then I read in above post to change the 02 sensors first. Can anyone who has had this problem get me headed in the right direction?


What is this "CC" that you're referring to? The P0420 code is not related to a gas cap issue.
The P0420 fault code is triggered by the catalyst system efficiency being below threshold. Here are some of the possible causes:

- The three way catalyst (manifold) is mechanically defective or poisoned.
- Exhaust pipe may have a hole.
- Intake system air leaks.
- One or more fuel injector defective.
- Fuel injector leaks
- Dirty spark plug(s).


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## missouriupser (Oct 6, 2019)

Catalytic converter- I had the manifold changed in May that has a catalytic converter built with it, it had an exhaust leak was reason I changed it out.


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## missouriupser (Oct 6, 2019)

Do you know if this particular car has two Catalytic Converters? I've read where there is an upper and lower but someone said that was for cars in California. This car was purchased in Arkansas originally and is in Missouri now.


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## rogoman (Dec 16, 2004)

If the engine is a QR25DE, then it has two cats in the exhaust manifold and one cat under the floor.


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## missouriupser (Oct 6, 2019)

rogoman said:


> If the engine is a QR25DE, then it has two cats in the exhaust manifold and one cat under the floor.


How kind I find this information on the engine?


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## rogoman (Dec 16, 2004)

Download a copy of the Factory Service Manual (FSM) from https://diyservicemanuals.com/ or https://ownersmanuals2.com/


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## smj999smj (Jan 1, 2006)

missouriupser said:


> How kind I find this information on the engine?


If it's a 2008 and it has a 4-cyl. engine, it's a QR25DE. If it has a V6, it would be a VQ35DE. You also should be asking in the "L31 Altima" section rather than the "L30 Altima," which is the previous generation to yours. Most of the time one gets a P0420 code on a QR25DE-equipped Nissan and there are no intake or exhaust leaks, no oxygen sensor codes and no drivability issues with the engine (i.e. rough idle or surging, misfiring, etc.), it's usually due to an inefficient, upstream catalytic converter...which would need to be replaced to fix. Many catalytic converter suppliers will want you to replace the oxygen sensors when installing the new converter to enforce the part warranty.


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