The Nissan Altima is a mid-size automobile manufactured by Nissan, and is arguably a continuation of the Nissan Bluebird line, which began in 1957.

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0303 Service Engine Soon code

0303 trouble code on my 2006 Nissan Altima. Leaving work I noticed a clicking noise coming from my engine and my service engine soon light come on. I had to make a stop on the way home and when I started my car again the click turned in to a load knock at first and silence down a bit 5 second later. I had a 5 minute drive home where I noticed the low knock got louder when idling at the lights. Once home I got the service engine soon code of 0303, which I looked up and found said it was a misfire in cylinder #3. I changed my coil pack for cylinder #3, and changed all my spark plugs, changed my oil, and checked all my fluids. In changing my spark plugs I noticed oil on the bottom of my spark plug from cylinder #1. After all that I still have the knocking. Other than a head gasket and valve issue, could I be over looking any other causing issue?

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Anytime you have an oily spark plug face, check your valves. Do a compression test as well as a leak down test. The knocking you are hearing, could easily be your valvetrain, camshaft, or your rod bearings. Especially since all this started before your OBD showed a 0303. For now, I also recommend that you drive as little as possible to avoid any further damage to your engine.

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Knocking is usually caused by slow ignition or because ignitions is too soon or too late. One of the easiest things to check is to make sure the gap on your spark plugs is at manufacture's specs. Depending on the mileage, another issue you can have is your camshaft and crankshaft are out of time. This happens when the timing belt (which is made of rubber) has more slack than the tentioner can take up. One way to check if its a head gasket, look for oil in your coolant or water in your oil (wierd spots in coolant or a white-ish looking sluge or goo in oil).

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