Bank 1 is the right side exhaust, and Sensor 1 is before the cat. The manual indicates it's detecting a lean condition. I'd clean the MAF sensor wires first. A dirty MAF sensor has been known to throw codes for a number of us here. I clean mine once a year. Then erase the code again and see if it comes back.
From the '99 VX repair manual for DTC P0131-
Mark
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