Chet's Auto Repair Inc Had a noise and a check engine light. Lewisville Texas

I had a check engine light with a code 0420 and a noise. The mechanic replaced the alternator and used his little machine to turn off the check engine light. The day after I picked up the car the light came back on and I still had the same noise. I went back over to the shop and showed him the code that the car gives you when you turn the key on and off 3 times. The same code P0420 that I took it to the shop for to begin with. His initial evaluation stated that # 3 cylinder was misfiring alternator caused poor performance. 13 days later he called to say my car was ready. I went by on 3 different occasions and his shop was closed at different times of the day on different days of the week.