My 96 used to do the same thing at idle after so long with ZZ4 cam, it ran great but after idling so long it would set the same code at idle, so if it runs fine just turn off the code, if don't ou need to see what cylinders are showing a misfire with a scan tool and go from there. I also showed this code on my 01 pu after putting in my LS1 I copied over all the fuel tables for a LS1 and flashed my computer, except for the MAF which was used the orignal off my truck. Code 300 would always come up around 80 mph, so after looking at info on scanner I decided that program was lean so I used the program off the camaro as far as programming go, it was to rich according to scanner at low speed so I blended it in program on the MAF tables and no more code 300, trims are closer now and no codes, hopr this helps.

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