2001 Chevy Blazer LT 4.3L V6 98,000 miles

Service Engine Soon light is illuminated. Took truck to a repair shop for a diagnostic and trouble codes read: PO131 - Bank 1 Sensor 1 and PO151 Bank 2 Sensor 1.

I had the O2 sensors replaced a year ago, along with the air injection pump and check valves due to a failed emissions test.

What keeps causing this to happen? Bad parts or something much bigger?

thanks.