97 code help

[tkhilliard]

hi all got a 97 lx with 75k, 3.5l
engine light kicked on today and i get 12 43 55.
whenm its cold it runs perfect but when it warms up it runs and drives but at idle the car wiggles around like crazy, tach dont move up and down that much, a little at times but nothing huge.
i get code 12, i've read that will pop up and dont worry about it, but the 43 stumps me. any help in a direction to go will be greatly appriciated, especially since im broke

also gas milage has sucked for awhile now, sometimes 300 plus a tank then next time 220. the idle has been allittle rough for a while, but now pretty bad and a light