P0136, O2 Sensor circuit low voltage, specifically refers to bank 1, sensor 2, but has an identical counterpart for bank 2, sensor 2, and is also basically the same in regard to O2 low voltage codes for sensor 1.
This code monitors the oxygen sensor voltage and when it sets, it means that the PCM sees that it has remained low for over a predetermined amount of time, usually 2-4 minutes, but that varies depending on manufacturer.
When an O2 sensor is operating normally, it produces a voltage between .1 and .9 volts. A lean condition produces low voltage and a rich condition produces high voltage. In order to keep the engine operating efficiently, the PCM continuously and rapidly adjusts the fuel mixture in response to the O2 sensor signal.
A healthy O2 sensor will react to the change in fuel mixture by rapidly returning its own signal to the PCM. It’s basically a way for the PCM and O2 sensor to hold a discussion about fuel mixture. The PCM says “hey do this” and the O2 sensor replies “hey you just did this.”
The final result with a healthy O2 sensor is a rapid up and down switching of voltage.