O2 sensor DTC P0031

P0031 - Heater control circuit low (bank 1 sensor 1). Easy item to test, unplug the sensor connector for the upstream O2 on the drivers side. Look at the connector and you will see two wires that are the same color and two that are different. The two same color wires are the heater circuit. Those carry twelve volts to the O2 sensor to make it heat up and respond faster. Use an ohm meter to check for continuity between those wires. Also use a test light to see if the twelve volt feed is there as well.

https://www.2carpros.com/articles/how-to-use-a-voltmeter
https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

It is the first sensor in the exhaust pipe. On the V8 it's in the manifold, in the V6 it's in the pipe just above the convertor.

B1 means it is on the side where cylinder number one is. S1 means it is the first sensor in the system.

Yes BUT there will be a Y pipe coming from the convertor to the engine. In that pipe or in the exhaust manifolds themselves there will be a sensor on each side you need to be sure you check the sensor that is on the side where cylinder number one is. That will be B1 S1