# P0300 Code



## Segfault (Feb 1, 2013)

A friend of mine owns a 2005 Altima, 200k miles on it. He cannot pass the state inspection because P0300 code keeps appearing and disappearing. Is there anything that commonly causes this code on these cars?


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## smj999smj (Jan 1, 2006)

P0300 is a random misfire code, which is pretty general. It would be more helpful if it at least identified the cylinder. Anything that can cause a misfire can potentially trigger this code. Some of the more common problems on these cars include bad ignition coil(s), bad cam position and crank position sensors, and bad head gaskets. Spark plugs are due every 105,000 miles; has he checked them? A lean condition, which can be caused by several things, can also cause it.


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## Segfault (Feb 1, 2013)

Thanks! We decided to replace intake gaskets next, spark plugs and coils are new already. I read up on this and it seems to be time consuming. What gaskets we need? To make sure we do not have to leave the garage and go shopping.


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## smj999smj (Jan 1, 2006)

Intake gaskets are not head gaskets and there is a lot more labor involved in replacing a head gasket. A "head gasket set" would have all or most of the gaskets needed to do the job; you would also need RTV silicone form-a-gasket, coolant, engine oil and filter. You'll also want a couple of cans of brake cleaner to clean oil off of the parts. I would recommend the head be sent out to a machine shop to be tested for cracks and straightness. I also wouldn't replace the head gasket unless I knew for sure that it had failed. A cylinder leakdown test would be the best way to do this. As far as just replacing the intake gaskets, while they were a common problem on the older Nissans with the KA24DE engine, I have not heard of any failures on the later models with the QR25DE engines. To test for a bad intake gasket, you can spray carb cleaner around the gasket with the engine running; if you hear a surge in the RPM while spraying, there is likely a leaking gasket.


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## Segfault (Feb 1, 2013)

I never thought of head gasket, just intake gasket(s). How many is there? How to determine if the engine is KA24DE? I used to do lots of wrenching, even owned a small tune-up shop. But this was more than 15 years ago ... never worked on this Nissan engine.


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## rogoman (Dec 16, 2004)

Segfault said:


> I never thought of head gasket, just intake gasket(s). How many is there? How to determine if the engine is KA24DE? I used to do lots of wrenching, even owned a small tune-up shop. But this was more than 15 years ago ... never worked on this Nissan engine.


The 2002 - 2006 model series used a QR25DE 4 cylinder or a VQ35DE 6 cylinder engine. To verify the engine, the VIN label plate will start with a "1N4", then followed with either a "A" for the QR25DE or "B" for the VQ35DE.

First thing you might want to do is look at each spark plug tip to see if there's any fouling occurring which would point to the problem cylinder. Just like SMJ suggested, first go through the diagnostics before you start tearing the engine down. 

Are you using the OEM NGK spark plugs? A lot of folks use Bosch spark plugs because they are cheaper; Nissan engines don't run well on Bosch plugs.

Here are some possible causes to trigger a P0300 code:
- Improper spark plug
- Insufficient compression
- Incorrect fuel pressure
- The injector circuit is open or shorted
- Fuel injector(s) dirty
- Intake air leak


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## Segfault (Feb 1, 2013)

Thanks! 

I will inspect the plugs (intended to do it anyhow),
will check compression,
will check fuel pressure,
will check for intake leaks with starting fluid,
will inspect injector circuits,
will recommend taking injectors into a shop for testing/cleaning.
And will report back here.


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## Segfault (Feb 1, 2013)

Alright, 1-3 spark plugs are light brown and the fourth is white. Is it running too lean?


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## rogoman (Dec 16, 2004)

#4 cylinder might be running lean. There might be a vacuum leak at the #4 cyl intake manifold position. A good way to check the intake system for a vacuum leak, is to attach a vacuum gauge to a full vacuum source. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be 21 InHg.

If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle valve and at the MAF for cracks or loose clamps.

It might also be possible that the #4 fuel injector is not working correctly or may be dirty. Run some good injection cleaner, like Techron or Redline SL-1, through the system; Give the cleaner about a week or two to do it's job.


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## Segfault (Feb 1, 2013)

Thanks! The message you have entered is too short. Please lengthen your message to at least 10 characters.


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## Segfault (Feb 1, 2013)

Solved. Injector cleaner did it. Thanks everyone for your input.


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