I got a 2000 civic ex and get CEL for low voltage O2 bank 1 only during cold start ups. After I clear the code it never comes back till like the next day when I start the car before going to work, or when leaving work going home.
Dont know why it would say low voltage ?. I checked the wires on the 02 and they seem good, no dammage there.
Im wondering if it has to do with the PLX I tapped at the ECU ? I didnt use any butt connectors, I soldered these wires and taped em nice, hum ? Anyone have any thoughts for low voltage 02 bank 1 issues..

 

which wire did you splice into?
also the o2 sensor is not putting out enough voltage due to
the plx you installed...you have to remember that an o2 sensor is a voltage generator, and if disturbed it will malfunction...
hope that helped...take the plx off...lol

oh and the reason it comes back on later is because it has not completed the drive cycle for obdII readiness.