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No complaints, here. I absolutely enjoy driving my 2013 500 POP (Automatic – 104,420 miles – purchased used, May 2018, from FIAT dealer in Utah).
The check engine light came on in November, with barely noticeable misfire while idling, and error code P0301. I replaced all 4 spark plugs with NGK iridium and swapped in a new coil at cylinder one and four…no change in code reading. I replaced all four spark plugs, again, with new OEM NGK Copper, replaced the pcv valve and related washers/o-rings. I could keep the CEL off as long as I kept the engine revved over 2000 RPM (a trick I did for one whole day of driving). As soon as I let her idle normally, the light came on…and stayed on.
Time to check the injectors? The only test I was brave enough to do was electrical resistance at injectors with connector unplugged and car off. No variation was found.
I’m seeking the next step in diagnosing this problem – Unless someone can jump me straight to the fix.
Thank you!
 

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Dumb question, but did plug 1 look any different and did you check compression? The diag says it could be head gasket, low compression, vacuum leak or injector. I was thinking that might give an indication. Any secondary codes? (I hate it when codes simply say, somethings wrong in #1.)

They suggest resetting the computer. If it's the coil, maybe it'll move to #4?
 

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Each misfire can have different causes. Any trouble codes showing? Is it missing only at idle? First thing on an Abarth can be to change the plugs, especially if it's been over 25K. Follow on by some diagnostics. Long and Shot term fuel trims (LTFT / STFT) are something a handheld scan tool can give... should be +/- 5% for an unmodified car. Compression and leak down tests can give an idea of the valve and ring condition.

That can be a good start.
 

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I think since you've already done the easy bits (plugs, coils, etc) it might be time to start digging into the top end with compression tests. Also have your oil tested by Blackstone Labs.
 
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