Fuel Injector Replacement
 

I am experiencing misfires on a 2007 Fit with 105,000 miles on it. Got codes on cylinders 2, 3, and 4 with cylinder 3 indicated as primary culprit and suggestion that most likely solution was fuel injector replacement. I filled the gas tank with 89 octane am about half full on a new tank of 89 octane after routinely running 87. Added a can of Seafoam and changed the engine air filter which was not very dirty, however. (Plugs had been changed earlier to Autolite double platinum and ran fine for a while with those. It was suggested by someone that I should install OE irridium plugs, but it is my understanding that the benefit of those is only re longer life of irridium plugs). If any effect, car is running worse. I switched coils on cylinders 1 and 3 and got CEL with code only on 3, with an indication again that was probably a fuel injector problem. Tested firing on 1 and 3 coils with a test light which showed ignition on both. Car is going to shop for analysis/repair tomorrow. I have obtained a remanufactured fuel injector on the assumption that this is the problem. The labor cost for installation would be $165. The remaned part is $31. My concern is whether i ought to replace all of the injectors at the same time as it would cost another $165 labor if additional ones become necessary. I guess I have the answer to that as it would probably take a few days to get additional parts and "if they ain't broke...". I realize that there may be other explanations for the problem, and presumably i'll get an answer on that tomorrow. Any thoughts on the matter beforehand or things to consider or test?

If car runs worse after spark plug change, reinstall original plugs
(Use of any other than listed brand or type of the plug will affect engine performance; this may happen after weeks or months!
The old plug is still in working condition if the iridium tip ("electrode on electrode") is visible and not missing
It is detrimental to make sure that the base of the spark plug tube where the seal washer is siting is spotlessly clean/ free from metal scoring or the plug will leak.)