10 percent drop in gas mileage. Bad coil?
 

2013 Accord EX-L, 2.4L

Only 32,000 miles.

About three months ago my gas mileage dropped a very noticeable 10 percent or so. It had been quite good after a very studied (internet) and careful break in over a period of time: Five months ago I averaged 41 mpg driving from DC area to Kingston, NY....measured both by the car's mpg meter and by topping off the tank at the beginning and end of the trip to be sure.

Today I decided to put in new spark plugs, despite the low mileage, to see if that might be the problem. The white ceramic around the trip of the firing point of one plug is noticeably darker than the others though not oily/wet.

I suspect that cylinder is the problem, possibly the coil. A mechanic friend suggested I swap coils for that plug and one of the others and check it after driving a couple of weeks. I might try that. If the coil was completely shot I'd expect rougher running and a much dirtier plug. I'm thinking it's weaker than the others or there's some other problem with that cylinder.
https://cimg1.ibsrv.net/gimg/honda-t...36d0518931.jpg
Questionable plug on the left.

Also, I bought the 8 yr extended service agreement from the dealer so I still have almost two years of coverage and assume something like this would come under that umbrella.

Which way to do this? (I have a hunch I'll have to pay a diagnostic fee for the dealer to check it out and IF the problem is covered by the warranty that would be fee could voided. Am I right?)

Ideas or guidance? Thanks....