98 Codes

I have a 98 Dakota with a 3.9l V6. The engine light is on all the time. I went to Autozone and had them check it with a code reader. The code number is "P0420, Catalyst efficiency below require level." Does anybody know what this means?

you have a bad catalytic, a bad o2 sensor or circuit..What happens is the 1/1 o2 sensor before the cat is you fuel control o2 this o2 switches anywhere from .01v to 1.0, it tries to maintain an average of about .45volts. The o2 in the rear after the cat should maintain a steady voltage. if the rear o2 sensor sees the same varying voltage as the front is throws a code thinking that the cat converter is not working

How do you determine what is bad without throwing parts at it? Is there a way to test each sensor or could it be something else in the fuel system?

use a scanner. or you could use a heat temp and check the temp before the cat cand after the cat