P1131 - Lack Of HO2S11 Switches - Sensor Indicates Lean
It could be the sensor but usually this means the mixture of the car is lean.
Or at least the ecu is thinking the mixture is lean.

It can be a small leak in the exhaust near the Oxygen sensor - but then the car would be using a lot more fuel. Otherwise you are looking at a defective sensor or a blocked fuel filter.