Here's the thing: most cars will never need any work done on the injectors for their lifetime. Some will, and you will know when you have some issues with the enginew not running as it should. On a high milage car, it's is very possible that one injectors may not function 100% at every RPM. I have seen injectors on a test machine, and they can fire fine and idle, then mis-spray and mid RPM, then fire fine at WOT. No way for a car owner to know without looking at them on a test machine. If they are selling the injectors cleaning as routine maintenance, then that is likely a cash cow for them, but it is like buying insurance for the owner. an e-test can show bad injestors, but not always, and it can be many other issues that cause and e-test to read high. I think all fuel comes with an fuel system cleaner, so puttin it in every tank full is a waste of $$