How to tell which O2 sensor is bad on a Infinity QX4?

To determine which O2 sensor is bad on an Infiniti QX4, start by using an OBD-II scanner to check for trouble codes. Look for codes related to O2 sensors, such as P0130 or P0135, which indicate issues with the sensor performance or heater circuit. Additionally, monitor the live data from the sensors; a malfunctioning sensor will typically show a flat or erratic voltage reading while the good ones fluctuate as expected. Inspect the wiring and connections for any visible damage or corrosion that could also affect sensor performance.