What is trouble code P1131 on a 2000 Ford Taurus?

This indicates that your left pre-catalytic converter Oxygen sensor is bad.

P1131 and P1113 are pre-cat sensors, Bank 1 & Bank 2 sensor 1. Please refer to the diagram in the link below.

http://forums.audiworld.com/s4/msgs/2770872.phtml

They are anywhere from $70-200+ dollars and should be changed in pairs.