Locate the O2 sensor on the exhaust system
Inspect the sensor and wiring for damage, corrosion, or loose connections
Check for stored trouble codes with an OBD2 scanner
View live O2 sensor data on the scanner
Start the engine and let it reach operating temperature
Observe sensor voltage or air-fuel ratio readings
Verify the reading switches or responds quickly as engine speed changes
Check for a fixed, slow, or abnormal signal
Test the heater circuit with a multimeter if equipped
Measure sensor resistance only if specified by the vehicle manufacturer
Check for exhaust leaks before the sensor
Compare upstream and downstream sensor behavior
Replace the sensor if readings are faulty and wiring or exhaust issues are ruled out