need help with wifes Mercury

my wifes 96 merc. sable Ls,is running real rough, I believe it has a 3.0 duratec
so I took it to the auto parts store and they put that little gadget that
reads the codes,this is what came back( p1518 manf cntrl veh.spd. idle speed control auxiliary Inputs) (po420 catalyst efficiency below threshold..bank 1 ) (po301 cylinder misfire detected ) (po303 cylinder 3 misfire detected) (po171 system too lean bank 1, and the guy that works on my cars is out of townfor 2 weeks, is this something that one of you guys that have worked on cars with codes can help me solve, the guy at the parts store said he didn't know what the code p1518 was, I'm hoping on fixing this my self if possible so any information would really help, thanks..:)

thanks Denny, for the great help, I am going to try to check or replace
plugs and wires today,I say try cause it hard to get to the back bank (plugs)
but where and what do they look like this ?(Assuming the wiring is good and you replaced the IMRC Actuator, it is a good bet the PCM is bad.) I did notice
the plugs on front bank were wet and just finger tighten when I checked
them yesterday:confused: thanks again and I knew I could on you!!:)