When troubleshooting a faulty front oxygen sensor on a Toyota, it's important to check both the wiring harness and the sensor connector. A faulty sensor can also cause fuel trim and DTCs issues, so it's important to diagnose the issue correctly.  To use a multimeter to determine if the sensor is bad, you can follow these steps: Start by disconnecting the sensor connection and setting your test meter to the resistance setting. Measure the resistance between the two heater wires. The resistance should be between 10 and 20 ohms at room temperature.