hiu i have a 92 sc 400 my check engine light went on the dealer said it was the o2 sensors after the cat. they want $700 i found some on a web site oxygensensors.com for like $50 they are universial will tyhis be harder to install. how difficult is it to install one? next my valves are knocking i tried to get some info on this aswell. the dealer wamts $700 for this one to. how difficult would it be to fix this myself. o and one more thing do those cleaner things you add to your fuel actully work at all?