How to test my oxygen sensors on my 02 Honda Civic LX?

I recently purchased a 2002 Honda Civic LX (141000+ km) from this lady and took it to get a diagnostics test and the code P0420 came up (Catalyst Efficiency Below Threshold - Bank 1).

I'm hoping that it is just the oxygen sensors but I need to know the amount of voltage that should come up when you test the sensors and if there's another way to test it without using a flame torch.

Also might need to check the spark plugs… What kind of wrench will I need for that (they sit down so far!).

P0420 means the catalytic converter is below efficient limits, not the O2 sensors.
The best ways to clean I catalytic converter I have found is with a torch after dropping the flange at the front. But this video has some ideas.

Here is an O2 sensor test video:

From.1 to.8 volts.
The 3 wire type has a heater.

For the spark plugs, a long deep well socket is easiest. You can get one from a place like Harbor Freight, cheap.