It's probably getting time to change the plugs anyway, but let me throw out there the symptoms and get a consensus from the masses on here. Regular fuel regardless of octane, I've tried tanks of 87-93. Under hard acceleration I get #8 cyl mis-fire code and sometimes #3 but rarely. If I run Flex Fuel and keep the tank 51% or more Ethanol, no mis-fires what so ever BUT when idling in traffic for long periods or just idling in park, i get a check engine light for catalytic converter out of range (I assume this is due to the alcohol burning so clean there is not enough difference from front O2 sensor to the rear one??). On the OBDC the cats do run very cool on flex fuel vs pump gas. Last year I had the very common issue where the heater hose gets a leak in that T connector so the COPS got soaked and might have been damaged. Ford mechanic friend said don't replace the COPS unless you know they are bad because a good portion of new ones are DOA. Another local mechanic said do both while you have them out. Any thoughts on why no mis-fires on Flex and why Flex causes Cat issues? Any way to ensure a COP is good? Thank you. Also MPG suffers GREATLY with Flex making the cost savings not worth it.