I have a 2000 seville sts, started car no issues, drove to store 15-20 minutes, got back in started car and ck eng light flashing.
proceeded to pull over turn off and restart , still flashing.
Drove 25-30 minutes at highway speed, light went off then back on. now idle is rough.
Scanned at a-zone, po306 came up.
changed plugs and boots, actually seems worse,
Can anyone give me some diagnostic steps other than parts replacement, in order to fix this.