2005 Mitsubishi 380 gt

Was my work car for the last 7 driving beautiful till I changed the sparkplugs 2 days ago now hows a funny idle engine light is now on and flashes the car drives normal after 40ks only when slowing down I dont know the problem and dont have the time or money to look into it even more has 2 days rego 1450$

A$1,450