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Narrow Band Oxygen Sensors

In 1976, Bosch first released an oxygen sensor to the automotive market. At that time only Volvo and Saab used them, and it wasn't until 1980 that these sensors started being used in US vehicles. Narrow band sensors are often (and usually), just called "oxygen sensors" because for many years there was only this one basic type of sensor. As of this date (2008), most cars still use this type of sensor.

It's called a narrow band sensor, because it can only detect a very narrow band of air fuel ratio. It's basically an on/off switch in that it will either read lean or rich, but it doesn't tell the ECU how lean or how rich the mix is. It communicates with the ECU via the voltage it produces. The voltage jumps up to near 1 volt, and down to near 0 volts. When you read the signal on your multi-meter, you will see the voltages flash across your meter's display too fast to be able to read it. Often it will appear that the voltages are between .2 and .8 volts or .3 and .7 volts because the meter cannot change fast enough to show the actual voltages.

These sensors can have 1, 2, 3 or 4 wires. I'm not aware of any narrow band sensors that have more than 4 wires. Despite the varying number of wires, they all work the same way. One of the wires will always be the signal wire. The other wires, if present, will be a combination of the following:

  • The signal wire is always present, often called "signal high", "sig hi" or just "sig". This wire will always run to the ECU.
  • A ground reference for the signal wire, often called "signal low". If present, this wire will also run to the ECU.
  • Heater wire, which when the engine is running, will be 12 volts. This usually runs from a separate relay that supplies voltage when the engine is running.
  • Heater ground. This will usually run to the chassis. Sometimes it goes to the ECU and the ECU will control when the heater runs by making or breaking the ground connection.

Rarely, some vehicles, particularly Chrysler/Dodge will add a voltage bias to the sensor circuit. The ECU will provide a reference voltage of 2.5 volts on the 'signal low' wire to the sensor. The sensor's output on "signal high" will be added to this reference, such that it will oscillate between 2.5 volts and 3.5 volts (instead of 0 - 1 volt). These are the voltages you will see if you measure these 2 wires with reference to ground. However, if you measure between "sig hi" and "sig lo", you will see the same signal you are accustomed to seeing with unbiased sensors. While we haven't seen any other voltages used on "sig low", there's nothing to stop some manufacturer from using a different voltage than 2.5 volts. There's nothing different about the sensor in this case. The only thing that's different is that the ECU is putting a voltage on "signal low". Note that our Digital Series EFIEs must be modified to work with these vehicles.

Note: Rare Type of Narrow Band Sensor

There's another type of narrow band sensor called a Titanium Dioxide sensor. We only see this rarely. I recently looked up a 98 Jaguar engine that had this sensor. It uses a voltage of 0 to 5 volts, and the logic of rich and lean are backwards from other sensors (low voltage means rich, and higher voltage means lean). The only EFIE that we know of that will work with these is our original Dual EFIE. It has a range adjustment that allows you to increase the range. Then, because of the reversed logic, you have to hook up the EFIE backwards by connecting the computer wire to the sensor and the sensor wire to the computer. This causes the EFIE to subtract voltage rather than add it, and will then work with this type of sensor. For more information see the Analog EFIE Installation Instructions.

Wide Band Oxygen Sensors

Wide band sensors are a newer type of sensor. They started being used on a few Toyotas in 1997, but were used more and more in later years. The early adopters of this type of sensor are basically the Japanese and German cars. They are just now starting to show up in American cars (as of 2008). The wide band sensor not only tells the ECU if the mix is rich or lean, but how rich or how lean it is. In this way, it's easier for the ECU to adjust the mix without a lot of overshoot and guess work. For this reason, I feel that the wide band sensor is a superior technology, and that its only a matter of time before they are used exclusively for determining air/fuel ratios in cars and trucks.

These devices also have a completely different method of communicating to the ECU. Narrow band sensors communicate their information by providing a voltage that is read by the ECU. Wide band sensors use a current flow, the strength of which is directly proportional to the amount of oxygen present in the exhaust stream. It is very precise. Further, the current flows one direction when the mix is lean, and other way when the mix is rich. It's for this reason that you can't use narrow band style EFIEs on wide band oxygen sensors. You need to use one that is designed to control current rather than voltage. You can get EFIEs for wide band sensors at our Online Store. There are 2 basic types of wide band sensor which are described in the following 2 sections.

Air Fuel Ratio (AFR) Sensor

This sensor has 4 wires. One pair of wires is for the heater. One of these will be a ground, and the other will be for 12 volts. Wide band sensors need to maintain a correct tip temperature, and therefore the ECU will control the amount of heat supplied by the heater circuit. Usually it's heater ground runs to the ECU, and then the ECU can make or break the ground, as needed, to maintain the tip temperature in an acceptable range. This differs from the narrow band sensors which just supply a constant 12 volts to the heater.

The other pair of wires will be the signaling wires. One of these wires will be a reference voltage supplied by the ECU. The other one will be about .3 volts higher while the engine is running, but will vary slightly while providing different readings to the ECU. The reference voltages we have seen on various vehicles are: 2.5V, 2.7V and 3.0V. But there's nothing to stop a vehicle manufacturer from using other voltages than these. These are just the ones we have seen. In each case, the signal wire is approximately .3 volts higher than the reference.

The AFR sensor is not giving a voltage reference to the ECU. Instead it's varying the current that flows through the 2 signal wires. If current flows one way, the mix is lean, and if it flows the other way, it's rich. The amount of the current tells the ECU how much it's lean or rich. The amount of current is relatively small (0 - 30 mA).

Because this sensor uses current to communicate with the ECU, a normal EFIE cannot be used with it. We have seen people try to use narrow band type EFIEs to control these types of sensor but it just won't work. We have also seen people try to sell narrow band type EFIEs and claim they work on wide band. This also is just not true. You must have a device that is designed to modify the amount of current flowing, not a voltage reference. Our Wide Band EFIEs are designed to do this. Also note that the current adjustment is very tiny. Usually you only need to change the current flow by 1 milliamp or so.

5-Wire Wide Band Sensor

The 5 wire wide band adds one more element to the 4-wire version described above. It has a 5th wire that gives a voltage representation of the current flowing in the 2 current wires. This type of sensor also comes in a 6-wire variety that uses the 6th wire as a signal ground for the 5th wire's signal. Narrow band types of EFIEs have been known to work on the 5 wire wide bands. However, there is no guarantee that the ECU is not monitoring both the current and the voltage. Our Wide Band EFIEs always work because when they modify the current, the voltage reference is also modified. Other than the additional wire(s), the 5 and 6 wire varieties work the same as the 4 wire wide bands described above.


This is not intended to be an exhaustive description of how every type of oxygen sensor works. It is more of a broad description of the two main types (and sub-types) of sensors that are found to be equipped on fuel injected vehicles today.

For more information on other aspects of Oxygen Sensors or the EFIE devices that we offer to manage and adjust them, please visit our other articles in our Knowledge Base.

For help in determining what type of oxygen sensor(s) that your vehicle is equipped with, check out this article, here.

Oxygen Sensor Types