Looking at Honda parts suppliers, there appears to be two O2 sensors. One before the catalytic converter and one after it.

Check:
-electrical connections from wiring harness to the O2 sensors.
-corrosion on the O2 sensor bolt might be causing the low voltage warning. (assuming the O2 sensor is a single wire connection)
-a bad ground could also cause the code. (but, I would guess both O2 sensors would show the same warning. not just one)

You would have to google " honda 2001 accord v6 bank 1, Sensor 1 O2 sensor" to find out which one is throwing the error.

Part sources:
-honda dealer (pricey. but, OEM)
-your local auto parts store (aftermarket that meet or exceed OEM spec)

maybe, use some conductive anti seize on the new O2 sensor bolt (make sure to avoid getting anti seize on the sensor itself; only the bolt threads)
You'll need an O2 sensor socket and probably some penetrating oil to remove the old O2 sensor.

take care,
ChrisS