What is fault code p0031 for 2003 tacoma?

Oxygen (A/F) Sensor Heater Control Circuit Low (Bank 1 Sensor 1)

symptoms
Other than the MIL light illuminating you may not notice any performance issues.

cause
A short in the O2 sensor heater circuit or the sensor itself
A faulty O2 sensor heater
Wiring or connectors are broken or frayed to the sensor or relay
Bad PCM or ECM

The fix
Correct wiring short or loose connector found in inspection.
and/or
Replace the bank 1 sensor 1 O2 sensor