P0138 shorted to volts

Bank 1 is driver's side. Sensor two is the downstream sensor, after the catalytic converter. That oxygen sensor can be shorted internally, but it is much more common to have wires melted and shorted together in the wiring harness, or there is corrosion built up between two terminals in the connector. The secret is to watch the signal voltage on a scanner, and see what it changes to when the sensor is unplugged. If you do not have a scanner, depending on the model year, the check engine light may turn off when you do something to remove the short, (unplug the sensor or wiggle the harness), or it should turn off after you remove the short, then cycle the ignition switch off and back on. I do not think you need to erase the diagnostic fault code for the check engine light to turn off, but if you do, disconnect the battery negative cable for a minute.