Q: Code P0304 (spark plug and COP already replaced - reset code and still have a miss on 4 cylinder.

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2003 F150 - 5.4L - Had an issue with P0306 - replaced the COP on cylinder 6 with an older COP - everything was fine. Ordered a set of 8 COPs and replaced all - everything was fine - about 200 miles and I started having hesitation - finally got code P0304 - switched COP 4 with CO P1 - reset code. Still had the same problem and received code P0304 still - changed spark plug - went around the block - seems ok - about 10 miles later it started hesitating and the code came back P0304. (have had issue in the past with vacuum hoses - and have switched them out - don't think that is the problem now - but not 100% sure where to go from here)

My car has 118000 miles.
My car has an automatic transmission.

If the fault is now pinned down to one cylinder, a possibility is a faulty fuel injector. Faults can be of various sorts: injector leaks, the injector driver in the ECM, the injector itself in terms of the electrical solenoid/circuit. But, there are other possibilities and all will have to be tracked down until the fault is found: Faulty oxygen sensor(s); Burned exhaust valve; Faulty catalytic converter(s); Poor compression in that cylinder; Defective computer (rare). To get this resolved, the recommended service is a misfiring diagnostic. If you request that service, the responding certified mechanic will get this diagnosed and resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.

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