My 2015 Fit (180K miles) started setting error codes, mostly related to the Oxygen sensors/Cat Cov. It also lit up 4 unrelated fault lights at the same time. Plus, my gas mileage had begun to drop dramatically. I was getting 35 mpg in the driving situation I had previously gotten 42 mpg. The hive mind here on the Fit forums said my injectors were the likely cause of the latter error notice. So I changed my injectors (used genuine Honda parts costing $287 with shipping), and while I was at it, I cleaned the backside of the intake valves with a "walnut blaster" (like a sand blaster but using ground up walnut shells. The blaster cost $40). I also replaced the PCV valve. It took about 9 hours all told. I looked at the old injectors with a magnifier. They looked awful. Some of the spray holes were completely plugged. Bleh.
After the replacement and cleaning, my Fit immediately began getting 45–50 mpg (as reported by the onboard mileage reporter). My wife got 54 mpg on a 18-mile trip driving 50-55 mph. We were astounded by the change. The engine also runs smoother now, and no further engine fault codes have appeared (after 1,500 miles). I have no idea why the car is now getting much better fuel economy than when we bought it new ten years ago. Maybe the injectors have been improved. Having a garage replace the injectors would cost a lot, probably $1K–1.5K. A trained mechanic could probably do the replacement and cleaning in about 4 hours (my guess). It's not a simple job, but there are instructions for doing the job here on the forum that make it possible for someone with tools and some experience. The price for genuine Honda injectors varies A LOT, so if you do replace them, shop around. A number of internet vendors sell them for about $300. I've seen them selling for around $1,000. I'm leery of aftermarket with this kind of part.
I thought it would be worth reporting my experience here.

 

Thank you for sharing that. Are you used to working on cars? That sounds daunting to me. I usually fill up with Chevron and even use Techron fuel system cleaner between oil changes and I still have diminished fuel economy.

 

I have always fixed my own cars, so I do have some accumulated experience and tools. But I'm not a professional. Replacing the injectors is not easy the first time you do it. You need a good set of instructions, which are posted by Sam Lin on the FitFreak website. Great instructions! In addition to the injectors, it is necessary to buy a gasket and a small pipe that connects the fuel pump to the fuel rail. It's sold by Honda as a kit. There a quite a few parts to take off in order to gain access to the injectors. The hardest part of the job for me was getting the little fuel tube snaked into place. My injectors were visibly carboned up and, in some cases, plugged. It was bad. But I buy cheap gas and only occasionally throw in a bottle of injector cleaner. Doing the walnut blaster clean out wasn't hard, just tedious and annoying. Some of the carbon cannot be blasted away and requires scraping with a screwdriver. Blast, scrape, blast, scrape... Each cylinder took about 20 minutes of work to get it clean.
I don't recommend taking on this job or having someone do it for you unless your fuel economy has dropped dramatically or unless the four-lights-at-once error shows up. I'm glad I did it, and my wife thinks I'm a genius.