well at auto zone i got 02 sensor and changed it drove for a week then light came on, think it was p0131 "low voltage ,bank 1 sensor 1" i returned it and got new one and it already came back on!!!
what could it be?

bank 1 sensor 1, low voltage

Causes



Faulty o2 sensor Exhaust leak near the rear sensor
Plugged catalyst
Short to voltage on O2 signal circuit
High resistance or open on O2 signal circuit

Possible Solutions



Replace faulty sensor
Repair exhaust leak near the rear sensor
Check for restriction in catalyst and replace as necessary
Repair short, open, or high resistance on o2 signal circuit