o2 sensor questions

[Dhorn33]

When I bought my 98 GMC truck back in early December it had an SES light so I took it to Autozone and had them run a scan. It came back with code p0141 - o2 sensor bank 1 sensor 2. I did not replace the sensor but this morning I went back to the store intending to buy a new o2 sensor and had them plug it in again to verify the code. This time it came back with p0157 0 o2 sensor bank 2 sensor 2? The SES light is not always on - in fact it seems to come on for a few days then go off for a few days. The truck currently has 151k miles on it - and I have no idea if any of the o2 sensors have ever been replaced although when I looked under the truck it looked to me like the bank 2 sensor 2 was different from the rest - possibly indicating either it was a replacement or perhaps the others were replaced on that one was not. I don't really want to spend unecessary money on sensors that still function perfectly - so does anyone have any input on this?

[getcrunk88]

if you really want to get into checkig the ones u have now look up how to test o2 sensors