WHAT CAUSES MISFIRE CODE

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THOUGHT I HAD GOT BAD GAS CAR WAS SPITTING AN SPUTTERING--PUT IN GAS PRODUCT FOR THIS GOT BETTER BUT STILL HAVE SOME PROBLEM WITH THIS WHEN FIRST START CAR OR AT A STOP ENGINE LIGHT ON CODE WAS FOR MISFIRE--CAR HAS 210000 MILES DO I NEED NEW SPARK PLUGS OR WHAT ELSE