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Not much really. You may get Knock (a pinging/tapping sound only when engine is under load, slow rpms, hot weather, A/C on, and you press gas fairly much). You usually don't feel it so much as hear it, but it can result in slower engine pickup under these load conditions. Knock is technically pre-ignition in combustion chamber before ignition system says to fire. With most modern low-compression engines it is not usually a problem. Higher Octane gasoline was specifically made to reduce/eliminate Knock in high compressions engines. If you are using 87 Octane, then Knock can be cured by using higher Octane gas (one tank full will tell story). This may be one way to tell if you actually have Knock problem. If you do have Knock problem, then maybe you (or someone) unwittingly advanced the Ignition Timing too much or used wrong procedure. But if you FEEL the engine miss rather more hearing Knock, it may really be something else wrong - Not Knock. Again, with most engines of today Knock is fairly uncommon (even with 87 Octane) if all other ignition components are properly functioning.
 

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'97 Altima set the 325 knock senser code for over a year and the car ran except for a odd periodic failure every 2-3000 miles-engine would stall, restart after a while but seemed to be cured with new spark plugs-go figure. No other codes set. Three days ago coming home it quit in the driveway-lucky me! Now it will start, idle rough for a few seconds at about 500rpm then die. If the gas pedal is pushed it will die-no codes. Fuel pressure is spec, spark at plugs-there is a "Buzz" when pedal is pushed-haven't located the source.
 
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