What to do about check engine light that won't stay off? on 2000 Saturn LS1

My check engine light is flashing, I had a tune up and it went off for about 1 week, I had the catalytic converter replaced and it was off for about an hour. It is vibrating so bad it feels as if the engine is about to fall out.

1 answer
get a shop to do a build up of the motor to see why it is missing.