How do you determine the signal wire in a 4 wire oxygen sensor?

To determine the signal wire in a 4-wire oxygen sensor, first identify the wires by their functions: typically, the sensor will have two wires for heating (heater wires), one for the signal (usually a smaller gauge wire), and one for ground. You can use a multimeter to measure resistance; the heater wires usually have low resistance, while the signal wire should show higher resistance. Additionally, consult the specific vehicle's wiring diagram or service manual for the correct wire color codes associated with the oxygen sensor.