Im sure the injector cleaner worked and made a difference, but remember the check engine light can come on for so many different reasons. I have the code reader/reset tool and it has already paid for itself with just one use. I had the check engine light come on, I retrieved the code, replaced the gas cap and that was it. Had I not had the tool, I would have taken the car to the dealer and I could have been told anything and possibly been ripped off. The code reader/reset tool is a good investment, even if you don't do the work yourself. you will at least know the reason for the light. I myself do all the work, but I understand there are those who don't. To those I suggest you either find a mechanic who you can trust, or purchase a fault code reader.
One another thing to keep in mind, there are alot of states now that are using the vehicle's diagnostic port when the car is inspected. So if the check engine light is malfunctioning and you don't know it, your car will fail inspection if it is subject to a diagnostic check.