The check engine light went on (97 grand marquis) took the car to advance auto parts they ran the code it came back as catalyst efficiency below threshold bank 1 p0420
The guy said it could be a bad cat or bad 02 sensors.....but he couldn’t give me a straight answer said I would have to take the car into a shop so they could scan it better...I thought that was the point of them having a scanner....anyway does anyone know exactly would this code means......I am thinking of just replacing the 02 sensors and hoping that’s it......what do u guys think.....