P0420 Error Code

[MFCC08]

Hey guys my dad had a question about his 2000 corolla.

He says, the first code was P0141, and he ended up replacing the O2 sensor after the cat. When he put it in, he wasn't able to get voltage to the O2 sensor heater. A week later he got the current code of P0420.

If he goes back in and rechecks the voltage on the O2 sensor heater, and there is still no voltage, how does he fix that?

Thank you!