"WBO2S!  What is that? Some kind of radio station?"

 The landscape of automotive Diagnostics and repair is in a constant state of change, and the latest component to experience this change is our old friend the oxygen sensor. At this point you may be asking "Why change something that has been around for over 20 years and seems to have been doing a pretty good job?" Well, up until now, it did the job but with today's tighter emissions and fuel standards we need to watch the air-fuel mix a lot closer. Wide Band O2 does just that.

  The  narrow band sensors was designed to achieve a stoichiometric (14.7:1) mixture under closed loop conditions, as this is the point where optimum catalytic converter efficiency is achieved. There are ways to guess your actual air/fuel ratio (AFR), but there is only one tool which can give you exact values. That tool is the wide band oxygen sensor. The wide-band sensor expands on the principle of the two-state sensor narrow band function by incorporating a second chamber, the pump cell. The Wide Band Sensors are used in the pre catalyst position in the exhaust stream. They look similar to the Oxygen sensors that you have been accustomed to, however there are some differences.

A Brief Lesson

   Back in the early days of OBD-I, we needed the oxygen sensor to report back to the ECM on just what kind of mixture we were getting in the exhaust stream. By getting this information to the ECM, we can adjust the dwell period of our injector or carburetor accordingly. The sensors themselves were usually single wire affairs that would cool off in an extended idle, causing some vehicles to go into open loop. To prevent this from happening and to help reach closed loop faster, heaters were added. 

  The criteria for checking the feedback of these sensors was how far it strayed from the stoichiometric voltage of 450 millivolts. Anything above that voltage, meant a rich mixture and called for the ECM to lean things out by reducing injector "on time". Conversely, if the voltage was lower than 450mv, there would be an increase in injector "on time".

  Although somewhat accurate, these sensors are only capable of telling us that we are out of the ideal 14.7:1. A two state sensor, meaning we are lean or rich and that's it. The sensor is also sampled approximately every 50 milliseconds (ms), so there is a lag time between checks of the sensor which can lead to gaps and inaccurate readings and corrections to the A\F ratio.

Where Wider is Better (Not just for Pontiacs, either!)

  The sensors for wide band systems are very similar to the ones that you are used to. They look a lot alike with a few exceptions. The sensors will have 6 wires connected to them (Heater power and ground, Reference high and low, and 2 wires  for pumping current.  The ECM is attempting to maintain a constant 450 mV potential difference between the reference voltage and low reference circuits by increasing or decreasing the abundance of oxygen ions in the exhaust gas sample chamber, an external electrical force is required to move the oxygen ions. That's where the 2 extra wires for the pump current comes in. The ECM relies on Pumping Current to move electrons, here's how; 

  The ECM supplies a pumping voltage to the ion conductive material that separates the exhaust gas sample chamber and the exhaust stream. As the pumping voltage, and thus the pumping current, increases or decreases, as well as changes polarity, oxygen ions will be moved from the exhaust stream through the ion conductive material into the exhaust gas sample chamber. Ions may also be moved from the exhaust gas sample chamber through the ion conductive material back into the exhaust stream. The ECM monitors the pumping voltage level and polarity required to maintain the 450 mV potential difference between the reference voltage circuit and the low reference circuit electrodes and uses this information to change the air/fuel ratio accordingly. This new commanded air/fuel ratio will burn in the engine cylinders, then pass the sensor, entering the exhaust gas sample chamber, and the adjustment process will start again. This process happens extremely quickly and thus makes the wide-range sensor much more accurate, fast, and able to detect greater deviations, rich or lean, from the stoichiometric air/fuel ratio.  

How is this different from before?

  The ECM used to keep an eye on the 450mv as stated above, now the pumping current, polarity and reference voltage are watched. A value greater than 1.000 indicates a lean air/fuel ration; conversely a value less than 1.000 indicates a rich air/fuel ratio. At idle the lambda value will typically fluctuate between .900 and 1.000. The advantage to this is the fact that the pumping current can be monitored constantly for better AFR control whereas before we had the lag time between samples. Trends in the AFR can be monitored closer than ever giving us cleaner air and better mileage. 
  So to put in a thumbnail sketch, the wide band O2 sensor is a conventional switching sensor combined with an oxygen pump. Oxygen is pumped into or out of the exhaust sample chamber in order to produce it's 450mv. The amount of oxygen required to achieve this is proportional to the oxygen content of the sample.

 Mik Stubing - Lead Instructor GMTCNY