Trouble code 0420

My son's 97 GA with 3.1 has been throwing trouble code of 0420 and my OTC monitor say it is a "TWC System Low Effecitency" I have no idea what a TWC is or what or where to start checking. All help is greatly apprecitated!

Catalytic converter is not working correctly......most likely, you need a converter....

If you have a scan tool that can plot wave forms, plot the pre and post O2 sensors....the pre should change rapidly above and below 450 millivolts....the post should move very slowly....if they "mirror" each other, then the cat is no good.....from experience, when you replace the cat, you should change the post O2 sensor too.....

I agree with Tech II. The ECM monitors the pre and post cat O2 sensors to verify proper operation of the cat. As he said, most likely the converter will need to be replaced.

thanks for the info!