Can just bad spark plugs cause your check engine light to flash ?

My car's check engine light came on than starting flashing. The car was also hesitating badly. I had the cat converter replaced because of emission problem a few months ago and passed the test. I brought my car to the garage today and they said it's not the cat. Said probably needs a tune up because flashing light means misfiring. They couldn't do it today. Made an appointment for Monday morning. My son changed the plugs in the meantime and said it should be alright because the plugs looked really bad. The light is still on though and I haven't driven it yet. Is it safe to drive like this. Just encase it's a bigger problem and I damage my cat ? It's a 1996 ford escort wagon .