04 Allroad Check Engine Light Keeps Comming On

I have an 04 allroad and the check engine light keeps comming on. I have had the vehicle in the shop once or twice a month over the last year. They did replace the O2 sensor but the light still keeps comming on. Now they are telling me that it is a fuel related issue and to try different brands of gas. They have even recommended adding an injector cleaner but I'm concerned about adding any thing to the tank because I don't want to void my warranty by doing so and I don't want them to come back and say you were not supposed to do that now you have to replace your injectors. Has anyone had this problem? if so what was the fix?