99 Blazer p420 code

P0420 means (Converter Low Effeciency)
The computer measures the difference between the up stream 02 sensor and the down stream sensor. If the difference is out of spec it'll set the code.

The only possible causes are:
One/more 02 sensors are bad
the sensor wires/connectors are bad
the converter is bad
there's a leak in the exhaust up stream of cat
Damaged exhaust pip up stream of cat
bad computer

If you've put new sensors and cat, then eliminate those from
the list and check the other possible causes starting with the easiest.

That's not correct. Voltage doesn't mean the sensor or wires are good. There has to be a specific voltage at a specific engine output that the computer then compares to a preprogrammed map and the up or downstream sensor.

Anytime one of these values is out of spec, the computer will tell you. And these voltage values vary every second.