P0135 code

Sensor 1 will be the first sensor on the exhaust manifold coming from the cylinder head. Bank 1 will be the bank where number one cylinder is. In the case of this engine it will be the cylinders closest to the firewall.

This code is for a bad heater circuit. It could be a bad sensor or a bad connection to the sensor. A simple continuity test of the heater circuit in the sensor could tell you which it is.
https://www.2carpros.com/articles/how-to-use-a-voltmeter

To test it you would simply unplug the sensors connector, then probe the two large wires in the plug, you should have a connection, if it is open the sensor gets replaced. If not then you will need to use a scan tool to activate the O2 heater circuit and test for voltage on the vehicle side of the connector.