Rough idle, cyl 7 misfire code, bad alternator & battery, all happened at once???

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JBravo1977

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On Monday my 2014 Tahoe LT 5.3L (126K miles) had a hard start, and idled rough in D, but not as bad in N or P. Later that day it wouldn't start. I got it started with my jump box, charged the battery that night and took it to a shop the next day. By then it was throwing a code, which I was told was a cylinder 7 misfire (P0300?). I had a similar problem with rough idle about 18 months ago and had all the plugs and wires replaced at the dealer. This solved the problem. Months later, at my next oil change, the oil change guy (not at the dealership) showed me that there were 8 wires and 7 plugs laying in that place in the engine bay that looks like another location for a battery. I assumed that the last plug was laying in a ditch somewhere, but now I wonder if #7 didn't get replaced at all. Is it normal to have a sudden rough idle with the electrical problems, and is there anything else that could cause a rough idle and this code if the plugs and wires are good? Maybe a vacuum leak?
 

89Suburban

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Spark plug is the first thing to check.
 

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