Is your check engine light on? Generally you pull the code to tell you which sensor it is, The code will have information like
P0030 (M) 1/1 O2 Sensor Heater Relay Circuit Problem detected in oxygen sensor heater relay circuit.
P0036 (M) 1/2 O2 Sensor Heater Relay Circuit
1/1 would be bank 1 sensor 1( That would be the first 3 cylinders the first o2 sensor from the engine)...
And generally It's an upstream sensor that goes bad That would cause poor gas mileage.
Here you can find all the diagnostic codes