Check engine light

You did clear the light when you unplugged the battery, but didn't fix the problem that's setting the code off. The light will come back if you don't fix it. P0420...you just have an O2 sensor that needs replacing...fairly common. However, if this is the sensor behind the catalytic converter, even if it's bad you shouldn't have any driveability issues, even with the light on. But if you have to have your vehicle inspected for emissions, you better replace it or you will certainly fail.

I figure it didn't say which o2 it was but I think might be the one after catalytic converter

Also so if the light doesn't come on it mean it fine right

I replaced my CO2 sensor, have a rebuilt engine, a rebuilt transmission, new catalytic converter, new battery, new front end suspension, new tires, alignment and my engine light is still on!! Please help me. Mine is also a 1999 Dodge Durango with the largest engine 5.9 4 wheel drive- does anyone know what it might be?

could be a faulty computer