C280 check engine lite came on and error codes 30,34,35 & 84 indicating lean mixture showed up. Car has 108k miles with original O sensors. My mechanic is unsure of the cause and suggested starting with replacing the front O sensor first to see if the codes disappear the and then rear one and then...who knows? I question the "hit or miss" approach but am inclined to think may be the cause since they are original.

Appreciate any ideas and thanks