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p0420 2006 Nissan Altima 2.5

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New User

Mar 8, 2011, 12:28 AM

Post #1 of 4 (8094 views)
p0420 2006 Nissan Altima 2.5 Sign In

I'd appreciate some insight on determining what to do with a friend of mine's 2006 Nissan Altima with the 2.5 liter 4 cylinder engine. His car currently has around 100,000 miles and I know this person is very lax with car maintenance. He hasn't performed a tune up, hasn't replaced any of the 02 sensors yet, the valve cover gasket is leaking a bit, timing chain is still original too along with all of the exhaust components. The car has thrown a p0420 code. From what I've been able to discern, Nissan's variable valve tech at times allows both the intake and exhaust valves to be slightly open at the same time to a certain degree and if the precat material has been damaged due to the engine running richly, backpressure causes particulates will get into the engine from the exhaust manifold and wear the insides out- possibly resulting in the need for major repairs or a new engine. On a side note, I'm curious if other makes are vulnerable to the precat material potentially causing damage to the engine- especially on Subarus. I currently own a 2007 Impreza 2.5i. I'm curious if certain makes are not know to be vulnerable to this, why?

If pressure check ok, I was thinking of replacing the precat with a catless header with locations on it for the 02 sensors. I however don't know how this could affect the engine- run lean or rich and it could damage the other catalytic converter too. I'd also prefer that the warning light goes out and doesn't stay on due to the mod if can be helped this way. Recommendations on this? New stock? There aren't any other codes that would indicate 02 sensors needing replacement specifically either. Is there a convincing way to prove to somebody if they are going bad or not? From what I've read around the 02 sensors get lazy over time and don't respond quickly possibly resulting in an engine the runs too richly at times.

Also, I'd like to perform a compression test on the vehicle, but have never done so. How many times do I need to crank the ignition in order to get reliable readings on the pressure gauge? I could use some clarification on this part.
He actually thinks that he'll be alright to not replace anything and he may still be able to drive it another 70,000 miles or so which I have a hard time agreeing with.
I appreciate your thoughtsSmile

Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Mar 8, 2011, 5:18 AM

Post #2 of 4 (8089 views)
Re: p0420 2006 Nissan Altima 2.5 [In reply to] Sign In

What you are talking about doing is against federal law and likely won't do a thing for your problem. I don't know why you're assuming it's a problem with a pre-cat. The PO420 is telling you that the cat is not doing it's job any more. You just need to replace the cats. It's not the design of the engine that did this. It's being so lax on the maintenance that causes the failure.


We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.

New User

Mar 8, 2011, 5:32 PM

Post #3 of 4 (8078 views)
Re: p0420 2006 Nissan Altima 2.5 [In reply to] Sign In

So if the precat is bad, a different code from p0420 would pop? I know it may be one of many possibilities, but I explained why I think this is a good idea because of the potential consequences for his engine, this circumstance is real and has happened to many others-
and me considering that he may not be on top of preventative mx items. How do you verify the o2 sensors are working up to spec still? If not, can I rely on a related code to pop up right away or does it have to reach a certain extent.

I've been told that he could still pass emissions with a gutted precat or aftermarket header on an Altima forum. I don't know for sure, but that's certainly something to consider- thanks for mentioning it.

I know what the code means and just replacing the cats does not guaranty a solution. If there's an underlying cause, the cat/cats could become unserviceable again.

What other things should I be looking at?

And how can I check and justify expenses for misc part/s through empirical data if possible?

Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Mar 8, 2011, 5:39 PM

Post #4 of 4 (8074 views)
Re: p0420 2006 Nissan Altima 2.5 [In reply to] Sign In

First off, the pre-cats are not monitored independently so you have no way to know if it's the pre-cat with the problem or not,..... likely not. The Cat monitor is located after the main Cat.

Cats die from contamination like raw fuel, burned oil and coolant that passes through them. If you want your cat to last longer, just keep the engine running at optimum.


We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.

p0420 2006 Nissan Altima 2.5

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