Changed O2 bank 1-2 and check engine came back on. What must I do

What is your question?

2003 Mitsubishi Montero sport 3.0L. Obd scan tool said O2sensor bank 1-2. We replaced the sensor. But evidently the check engine light came back on. Mechanic said it maybe faulty wiring. Obd showed no current voltage running through the O2 sensor. Someone help before we go spending all sorts of money before hitting the nail. Please. I'll greatly appreciate it and thank you in advance

What makes this problem better or worse?

Haven’t noticed

How long has your 2003 Mitsubishi Montero had this problem?

Since we purchased vehicle approximately 3 weeks