# Rough Idle + P0420



## jeffzzang (Jul 6, 2006)

Hey Everyone:

My 01 Altima has recently been experiencing a rough idle when the car is in drive (usually at a red light). I took the car to the mechanic several times, and he performed several fixes to no avail. He did a fuel system maintence, replaced the spark plugs and wires and also did a cursory cleaning of the throttle plate (that one was a freebie, hence cursory).

The cursory cleaning of the throttle plate improved the symptoms for a little while, but the rough idle came back. This past weekend, the service engine light came on and Autozone said its a P0420 (Catalyst System Efficiency Below Threshold (Bank 1)). Has anyone else experienced this problem and know what to do about it? I don't want to have to spend money to replace the CC. My car has about 80K miles on it and is an automatic. Thanks in advance.


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## LONDONDERRY (May 19, 2004)

Spray some carb cleaner around the intake manifold, if the idle quiets down, then your intake manifold gasket is leaking. Very common problem.


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## jeffzzang (Jul 6, 2006)

*Now it's P0420 and P0300*

I took the car to a different place today to get it scanned and it came up with some new information. It still has P0420 but it also lists the following:

DTC Pending(Codes)
P0300
Random/Multiple Cylinder Misfire Detected.

Does anyone know what the two code combinations might mean? Also, I've noticed that my car idles high (over 1000RPM) when in park or neutral. Last, if I'm going downhill and I take my foot off the gas, the rpms jump up and down. Any insights? Thanks in advance.


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## Darktide (Jul 29, 2004)

Frank is right,
Its not your catalytic converter. The original diagnosis is correct. Your catalyst efficiency is your cars ability to burn off fuel. It wouldn't point to the catalytic converter.

An unstable idle would point to a vacuum leak. Granted with all the new age crap it might've been hard to diagnose but its still my guess. Bring your car back to your mechanic and ask him to spray some carb cleaner around the intake manifold (specifically around cylinder #4) and he will probably get the idle to race. This will unfortunately mean that you need to have your intake gasket replaced. Please remember to go with either Nissan Factory quality or IMHO the best quality which is FelPro.

Do a search on P0420 and you'll find alot of this information. I think a few people even posted how much it cost them to have the work done and few more ambitious ones have posted how they did it themselves. 

As for the P0300... I'm pretty sure its one in the same. At least the description would lead me to believe so. With the vacuum leak you leave excess gas in the cylinder like I stated earlier, which would make the O2 sensor read that the fuel level in the exhaust is far too rich and that one or moreof the cylinders must not be firing.

HOWEVER, you're missing the best opportunity here. This is a perfect time to put a spark plug in your tail pipe and do the old flame thrower trick. You just can't pass up this kind of a chance to have a flamethrowing Altima.... it'd be awesome. :woowoo: 

Anywho, Good luck with this and keep us posted

Darktide


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## jeffzzang (Jul 6, 2006)

*Back to the mechanic*

Went back to the machanic this morning and I suggested the possibility of a vacuum hose or intake manifold gasket leak. An hour later he came back and he said the resistance is high on the spark plug wires he will replace the spark plugs + wires with better quality ones (covered under my warranty since he just replaced them a few months ago). He said he sprayed around the intake manifold but found no leaks (he knew about the altimas leaking around cylinder 4). I'm going to take it back tomorrow after they get the part and see what happens. 

[crossing my fingers] please god let this be the last of my car troubles!


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## Darktide (Jul 29, 2004)

I'm glad to see that your mechanic is familiar with the car. That's always a good thing. I'm kind of surprised about the plug wires having too high of a resistance. I replaced mine with Autolite wires. And a warranty for wires that's been longer than a few months.... wow... you have quite the mechanic on your hands. Congrats, rarely do you hear of someone who is so credible and nice. Good luck with it.

Darktide


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## LONDONDERRY (May 19, 2004)

Darktide said:


> I'm glad to see that your mechanic is familiar with the car. That's always a good thing. I'm kind of surprised about the plug wires having too high of a resistance. I replaced mine with Autolite wires. And a warranty for wires that's been longer than a few months.... wow... you have quite the mechanic on your hands. Congrats, rarely do you hear of someone who is so credible and nice. Good luck with it.
> 
> Darktide


Hey Jay been a long time since I'v seen you on-line.
Its always a good thing to find a honest mechanic, I'v yet to find one in New Hampsire, unless you can recommend someone. 

Not to sound pestimistic, but I would keep and eye on idle and manifold gasket issue, because sooner or later its going to happen. I enclosed a thread that discussed the intake manifold gasket leak issue some time ago. There's some really interesting comments and theories as to why it leak and most of the comments are from well knowledgeable nissan owners. 

http://www.nissanforums.com/general-altima-stanza-discussion/110604-intake-gasket-leak.html

Cheers
Frank


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## jeffzzang (Jul 6, 2006)

*intake manfiold gasket*

I just spent 3.5 hours at the mechanic while he replaced the wires and continued his diagnosis. It appears a different mechanic worked on it today and found a small intake manifold gasket leak! I think the guy felt bad for me since I've put so much money into it failed repairs, and quoted me at $344.00 for parts, labor and tax. I shoulda read up on this months ago when I started taking it into the mechanic. Hopefully this will be the end of my automotive woes.


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## Vince21 (Jul 11, 2006)

jeffzzang said:


> I just spent 3.5 hours at the mechanic while he replaced the wires and continued his diagnosis. It appears a different mechanic worked on it today and found a small intake manifold gasket leak! I think the guy felt bad for me since I've put so much money into it failed repairs, and quoted me at $344.00 for parts, labor and tax. I shoulda read up on this months ago when I started taking it into the mechanic. Hopefully this will be the end of my automotive woes.



That seems like a pretty good price. My girlfriend just had her intake gasket replaced and the dealer quoted her $600+. Luckily her extended warranty covered it and she only had to pay the $100 deductible. I'm sure you will be pleased wiht the results after the gasket is replaced. It will run like a new engine.


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## LONDONDERRY (May 19, 2004)

jeffzzang said:


> I just spent 3.5 hours at the mechanic while he replaced the wires and continued his diagnosis. It appears a different mechanic worked on it today and found a small intake manifold gasket leak! I think the guy felt bad for me since I've put so much money into it failed repairs, and quoted me at $344.00 for parts, labor and tax. I shoulda read up on this months ago when I started taking it into the mechanic. Hopefully this will be the end of my automotive woes.



You have an excellent price. Typical price is 500-600. May I suggest that since you'll have the gasket replacd that you have them replace the PVC valve as well; because its right there when they remove the intake manifold.

Good luck
Frank


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## jeffzzang (Jul 6, 2006)

*Problem Solved!!*

I just got my car back from the mechanic, and the problem is solved. The high idle in park/neutral went away and no more rough idle while in drive. The mechanic cleared the MIL light, but we'll see if it comes back. I suggest to anyone who has a circa 2001 altima with similar problems, not to waste any money doing anything else and have them check for intake gasket leaks first - it'll save you a bunch cash! Thanks for everyone's help.


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## jeffzzang (Jul 6, 2006)

The check engine light came on shortly after I got my car back from the mechanic. I checked the code today and it's P0420 again. The exhaust manifold was definitely the key to the rough idle. My friend has a 2001 Altima SE and he has the SAME rough idle problem I had.


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