why are the RPMs going up and down after i changed the starter?

i have an 06 accord 2.4 and it was in perfect conditions with the RPM, one day my car wasnt starting, and it turned out to be the starter. a friend changed it for me and to install it he had to remove the manifold to get to the starter now when i went to turn the car on there is a check engine code P0122 and the RPMs are high on 2000 on park and then it after 2 min. it goes up and down from 2000 rpm to 1000 rpm. Can anyone help?