Depending on your vehicle you may have standard or wide range O2 sensors.
Standard or "Narrow" Band
Narrow band O2 sensors are typically found on earlier OBDII compatible vehicles as well as post-cat on newer models. Typically these sensors have an output range of 0-1 volts.
Wideband O2 sensors are commonly used on newer vehicles upstream of the catalytic converter and will have a wider range - typically 0-5V. Wideband sensors may often be displayed using the prefix "WR" (e.g. "WR02B1S1") and will generally be displayed using one of the following units:
- Equivalence Ratio: Lambda, or λ is used to display the current Air:Fuel ratio compared to the ideal stoichiometric AFR. For a lambda value greater than one this means the current AFR is higher than the ideal AFR which indicates a lean condition.
- λ < 1.0 rich
- λ ~ 1.0 ideal
- λ > 1.0 lean
Note: this is the inverse of the fuel:air equivalence ratio ϕ
- Current: Similar to equivalence ratio but displayed in milliamps
- Positive Current Lean
- 0 mA ~ ideal
- Negative Current Rich
The bank number refers to which 'side' of the engine the sensor is associated with and usually corresponds to the location of cylinders #1 & #2.
Typical layouts (note your vehicle may be numbered differently):
- Inline 4 Cylinder (transverse & longitudinal)
Generally I4 engines will have a single exhaust manifold so you will only see sensors on bank #1
- Inline 6+ Cylinder
Some engines will have a single exhaust manifold/bank while others (e.g. BMW) will have two banks where bank #2 corresponds to cylinders 4 through 6.
- Transverse V6+ (most FWD, some AWD vehicles)
Cylinder #1 is generally the closest one to the 'front' of the engine, depending on the orientation of the engine your vehicle this could be the front or rear facing cylinder bank.
- Inline V6+ & Boxer (most RWD, some AWD vehicles)
Generally cylinder #1 / bank #1 are on the driver's side of the vehicle, although on some Audi/Ford V8s as well as Land Rovers and Subarus this may be reversed.
- Sensor 1
Sensor #1 is upstream of the catalytic converter on the exhaust manifold and used for monitoring and adjusting the AFR.
- Sensor 2
Sensor #2 is downstream of the catalytic converter and used to monitor its operation and efficiency. Typically this sensor is not used for adjusting fuel trim, if your vehicle reports S2 fuel trims they may appear as -99.2% which indicates "Not Used".
On a 2014 Chevrolet Silverado 1500 with the 5.3 V8, B1S1 would refer to the standard pre-cat O2 sensor on the exhaust manifold on the driver's side of the engine.
For more information on interpreting O2 sensor data Walker Products has a great training guide which takes roughly 30-60 minutes to complete.