O2 sensor

[crsgmc]

My CEL came on with a p0134 (O2 Bank 1 sensor 1). The truck is running fine. I ordered two new O2 sensors to replace both. I have already replaced both of the sensor1 sensors when I installed a new engine. That was 16,000 miles ago. After reading the codes, I cleared the code, realizing that the next time I drove the truck it would be back on. I have now driven over 80 miles and the check engine light is not coming back on. I have not replaced the sensors. Anybody else have this problem. Not sure if I should even bother replacing the sensors when they come in the mail.

A second question is that I have the 5.0 exhaust (single pipe). It has four O2 sensors, two precat one just before the catalytic converter and one after the catalytic converter. The first two bank1 bank2 sensor1 run the engine, bank2 sensor 3 is there to check for a correctly functioning catalytic converter. What is the bank2 sensor2 do? Is this a backup if one or both of the sensor1 sensors go bad?

Thank you for any advice.