Oxygen Sensor (O2 sensor)
Exhaust gas oxygen sensor (EGO or O2S) or Lambda sensor is the key sensor in engine fuel control feedback loop. The engine computer uses O2 input to balance the fuel mixture. Leaning the mixture when O2 reads rich and richening the mixture when sensor reads lean.
Rich Mixture (A/F over 14.7:1)=High O2 voltage out put (600~1000mV)
Lean Mixture (A/F under 14.7:1)=Low O2 voltage out put (300mV or less)
When A/F ratio is perfectly balanced at 14.7:1, it is called stoichiometric ratio, and sensor’s out put is about 450mV.
Down stream O2 sensors are used to monitor operating efficiency of catalytic converter. If converter is doing its job efficiently, there will be little unburned oxygen left in exhaust. So down stream O2 sensor gives steady out put instead of switching back and forth like up stream O2 sensor.
If down stream O2 sensor is switching like up stream O2 sensor, it means catalytic converter is not working.
Symptoms of failed O2 sensor
1) Failed emissions test (high CO or high HC)
2) Damaged catalytic converter
3) Poor fuel mileage
4) Fouled spark plugs
5) Engine runs rough
6) Sluggish performance