A mechanic changed my spark plugs and wires 7 years ago in my car with 3800 v6 don't know if it is platinum or iridium but it is Autolite plugs and wires. I only put 15k miles since then. It is running okay but I am only getting 16 mpg. I changed my upstream oxygen sensor, it looked ash color, I suspect the fuel injector cleaners caused my oxygen sensor to turn into ash color. Do you guys think my spark plugs would go bad by the use of fuel injector cleaners and with the age? .