Up until relatively recently, all oxygen sensors were of a type known as narrow band sensors. The reason these sensors are called "narrow band" is because they are only able to tell us if the air/fuel ratio is above or below a single known amount or a single narrow range. It can tell us that the mix is either rich or lean, but it doesn't tell us how rich or how lean the mix is.
Wide band oxygen sensors are also called wide range oxygen sensors, air fuel ratio (AFR) sensors, or just A/F sensors. They are called "wide band" sensors due to the fact that unlike narrow band sensors, they are not only able to tell the computer if the air/fuel mix is rich or lean, but how rich or how lean it is. It is able to signal to the computer a wide range of air/fuel mix readings. This makes it much easier for the computer to make adjustments to the fuel trim to achieve its targeted air fuel ratio.
These sensors are new, and weren't used in any vehicles prior to 1997. Starting in about 1999, nearly all Toyota models started using them. However, other than various Japanese and German makes, most automobile manufacturers have yet to adopt them. Because they are a superior sensor, it's probably only a matter of time before they are universally adopted by all manufacturers.
Wideband EFIE devices are designed to deal with these new sensors. We have found that it's actually much easier to control the air/fuel ratio of a vehicle with wide band sensors and our new product than with our earlier EFIEs on older style vehicles. But we still need to know the basics of how these sensors operate, so we can work with them.
How the Sensor Signals the Computer:
Unlike narrow band sensors that communicate to the computer by means of a voltage on a single wire, the wide band sensor uses two wires and signals the computer by means of a current flow. An air/fuel ratio of 14.7 to 1 (by weight), is considered to be the optimum air/fuel ratio. When the ratio is above this value, the current flows in one direction, and when it is below this value it flows in the other. When the air/fuel ratio is exactly 14.7 to 1, the current doesn't flow at all. In order to signal increasing rich or lean conditions, the current flow increases in ratio to how rich or lean the air/fuel ratio is.
The two wires we are discussing are called the current pump wires. The voltages on these current pump wires varies from manufacturer to manufacturer. One of the 2 current pump wires will have a voltage supplied to the sensor by the ECU. The other wire will be a return wire from the sensor to the ECU. Toyotas have 3.0 volts on their reference wire and the 3.3 volts on the current return wire. Note that the 3.3 volts will vary slightly as the current flows, but these changes are very tiny. Likewise, many Nissan sensors use 2.7 volts on their reference wire, and the current wire is approximately 3.0 volts. So far, in all of the 4-wire Wideband sensors we've seen, the difference between the 2 current pump wires has been a nominal .300 (300 millivolts), that fluctuates slightly based on current flow.
5-Wire Wideband Sensors
There is another type of wide band sensor that uses 5 wires, and sometimes 6 wires (rare). In this case there is a 5th wire that gives a voltage representation of the current flow on the current pump wires. When a 5th wire is used in this way, it will usually be called the "signal wire". The 6-wire versions also supply a ground reference for the signal wire. In both of these cases, there is circuitry to convert the current flow on the current pump wires into a voltage. But this type still uses the current pump pair of wires to control the voltage on the 5th wire.
The simplicity, as far as installation of the Wideband EFIE goes, is that we are looking for the two current pump wires, and we are attaching our device to the wire that has the higher voltage of that pair. If you find a sensor that uses voltages that are much higher or lower than those described above, you may have a misidentified wire or device. Double-check those wire readings to make sure.
The Heater Circuitry:
Wide range sensors require a tip temperature over twice as hot as narrow band sensors. The temperature also must be maintained within a predefined range. To achieve this, the 12 volts is pulsed to the sensor's heater, and the "on" time of the pulse is varied as needed to keep the temperature in the proper range. We've also seen examples where the 12 volts is supplied constantly, but the ECU makes and breaks the ground connection in a similar manner. Either way allows the ECU to control the exact temperature at the tip.
Identifying the heater wires can be a bit tricky. When the 12 volts is being pulsed, and you read the voltage on your multi-meter, you will not see 12 volts. You will see a lower voltage, like 6 or 8 volts for instance. This is because the meter is trying to give you the average voltage over a period of time. If you have a frequency function on your meter, you would see the frequency of the pulses to verify that you had the heater wire. If the ground is being pulsed, then you will see 12 volts on the heater wire, and a ground reading that might not be stable.
So far, every vehicle we've seen that uses wide band sensors, only uses them upstream of the catalytic converter. The downstream sensors have always been narrow band sensors. Further, with modern vehicles, we have found that you must treat both the upstream and the downstream sensors to be successful. The Wideband EFIEs are designed to manage both the upstream wide band sensor(s) and the downstream narrowband sensor(s).