http://www.obd-codes.com/p0131
"P0131 OBD-II Trouble Code
Technical Description
Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 1)
What does that mean?
Essentially the same as P0136 and P0137, a P0131 code refers to the first oxygen sensor on Bank 1. P0131 means the O2 oxygen sensor's voltage remained low for longer than 2 minutes. This, is interpreted by the ECM as a low voltage condition and sets the MIL. Bank 1 Sensor 1 is located in front of the catalytic converter."