Knock Sensor and O2 Sensor fault codes

[MisterH]

First time poster here and I've found a lot of useful help and direction reading the posts. Now I would appreciate some advice on two codes that have come up intermittently singly and together>

Have 99 QX4 with 152K. It performs fine and starts up easily. Last week the SES light came on and I used my iphone OBD tool to read the results.
Showed two errors: PO325 (knock sensor) and PO 161 (O2 heater bank 2, sensor 2)
In the chance that it might be just a temporary glitch I cleared it and it came back after a day but this time it only showed the PO161 error.

My question is, would the first place to start be in replacing that rear 02 sensor or should there be other items to check?
many thanks in advance for your input.

[rgk]

No. The rear O2 sensor has no effect on engine management. It is there solely for emissions diagnostics.

Most likely, your knock sensor is on the fritz. Have you experienced driveability issues? It is also possible the connector or wiring is acting up. Start by inspecting it.

[MisterH]

Thanks for your reply. As I mentioned earlier, the car runs just fine. Smooth, consistent idle, steady acceleration and easy starting. I cleared the codes twice in the last few days and now only the O2 sensor (PO161) error is appearing. I also notice the error appears after the car reaches optimal running temp, so is this not indicative of the heater circuit of the sensor failing?