How do you know which oxygen sensor to replace?

To determine which oxygen sensor to replace, start by checking for diagnostic trouble codes (DTCs) using an OBD-II scanner, as these codes can indicate which sensor is malfunctioning. Common codes for oxygen sensors include P0131 to P0135 for upstream sensors and P0136 to P0141 for downstream sensors. Additionally, inspect the sensor visually for any physical damage or corrosion, and consider the sensor's age and mileage; if it's been in use for over 100,000 miles, it may be due for replacement regardless of codes.