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Types of Oxygen Sensors

The first thing you have to know is that this problem is simple. There are only 2 basic types of oxygen sensor. These are narrow band sensors, and wide band sensors. We will cover these in detail below. There are a couple of oddball exotic types, but these just aren't used anymore. However, they will also be covered.

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.

Narrow band sensors are named appropriately 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. The following video will show what the signal wire looks like on a volte meter.

Narrow band sensors can have 1, 2, 3 or 4 wires. I'm not aware of any that have more than 4 wires. If you find you have more than 4 wires on your sensor, see the section on wide band sensors (below). 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:

  1. The signal wire is always present, often called "signal high", "sig hi" or just "sig". This wire will always run to the ECU.
  2. A ground reference for the signal wire, often called "signal low". If present, this wire will usually also run to the ECU.
  3. 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.
  4. 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.

Sensors with 1 or 2 wires don't have the heater wires. The o2 sensor needs to be hot before it will work. Since it is placed in the exhaust stream, it gets heat from the exhaust gasses of the engine. However, it takes time for the sensor to get hot that way. Heaters were added so that the sensor would become operational sooner. Sensors with 1 or 2 wires are no longer used much. Modern engines primarily use 4 wire narrow band sensors.

A Chrysler/Dodge Oddity

Rarely, for some Chrysler/Dodge models (only) in the 90s, the computer 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 have a switch so they can work with these vehicles. Even Dodge/Chrysler stopped using this format. Later versions (after the ealry 90s or so) don't use this format anymore.

Rare Type of Narrow Band Sensor

There's another type of narrow band sensor called a Titanium Dioxide sensor. 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 analog types. It has a range adjustment that allows you to increase the range. The normal range is 0 - 1 volt, but these sensors need 0 - 5 volts. 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 are sometimes called Air Fuel Ratio (AFR) Sensors, AFS or Air Fuel Sensors, or wide range oxygen sensors. These are all names for the same thing. 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 started being used in American cars in 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.

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.

Wide band sensors used in Toyota and Honda have 4 wires, which make them look like narrow band sensors. They label them as AFR sensors, but again, that's just another name for a wide band oxygen sensor. Other vendors mostly use 5 wires and sometimes 6. When you see more than 4 wires you can know that you are dealing with a wide band sensor. Like narrow band sensors, 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. Some narrow band sensors operate their heaters in the same way.

2 other 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. 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, but again, this is experiential data too and there's nothing to stop a manufacturer from using different schemes, so don't be alarmed if you see something a bit different than this.

Remember, the ECU is not measuring the voltage. It is measuring the current flowing through this pair of 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.

For more information on how wide band sensors work, I recommend this article: Wide Range Air/Fuel (WRAF) Sensors

How Oxygen Sensors Are Used

Oxygen sensors are used in 2 ways. The first and primary useage is to monitor the air/fuel ratio in the engine. Sensors for this purpose are always installed on the exhaust pipe in front of, or upstream of the catalytic converter. The cat is designed to knock out the bulk of the hydrocarbons in the exhaust so they don't pollute the atmosphere. But in so doing, they make the signal relatively useless for monitoring the air fuel ratio. So, these upstream sensors are the primary target for the EFIE.

In all vehicles after 1996, there are also downstream sensors. Some manufacturers used them in earlier engines as well. These sensors are primarily used for monitoring the function of the catalytic converter. If the cat is working properly, then the exhaust stream will be have more o2 vs hydrocarbons than if the cat were damaged or worn out. If that happens, the downstream o2 sensors will throw a trouble light, and may impair the functioning of the engine. These downstream sensors are always after the cat, or sometimes on the cat.

Upstream sensors can be narrow band or wide band. But, as of this date, all downstream sensors I've ever seen were narrow band. This doesn't mean that at some point in the future that wide band won't be used downstream. But as of 2018, I've never seen it done by any manufacturer. This means that EFIEs for downstream sensors are always of narrow band type. So, even if your car uses wide band upstream sensors, the downstream sensors will still be narrow band.

Normally V-6 and V-8 engines have 2 upstream sensors; one on each exhaust manifold. They usually, but not always have 2 downstream sensors as well. 4 cylinder engines usually have 1 upstream and 1 downstream sensors. I have seen where they have 2 downstream sensors also, but never 2 upstream. That doesn't mean you won't run into other configurations. But this is what I have seen.

We package EFIEs that are designed to treat all of the sensors on your engine in one package. We have a Dual And Quad Digital EFIE which work on cars with narrow band upstream oxygen sensors. The Dual Digital EFIE is for cars with one upstream and one downstream sensor. We also have a Wideband Quad EFIE. This is for all cars with wideband upstream sensors. The Wideband Quad EFIE has 2 wide band EFIEs for the upstream sensor and 2 narrow band EFIEs for the down stream sensors.

If you are using and HHO system, you will normally have to treat both the upstream and the downstream sensors. I've seen people succeed by treating the upstream sensors only, but this is usually on older cars and is far from universal. Often, and engine will throw error codes when it "thinks" the exhaust is "too lean", and will usually throw more fuel into the engine. So, it's best to treat both upstream and downstream sensors.

How to Determine What Type You Have

If you are going to use an EFIE with your oxygen sensors, you will need to know what type they are. Figuring this out can be easy and straightforward, or it can be tricky, depending on the make, model and year of your car. This section will help you work it out. The quickest and easiest methods are listed first, then the more complicated ones. Remember, the downstream sensors are always narrow band. So we are only figuring out the upstream sensor(s).

  1. The information might be written on a tag under your hood. Open the hood and look up. Many vehicles have a tag there with various information and many times it includes the type of oxygen sensors it has.
  2. The most easily identifiable form of wide band sensor is the 5-wire wide band. These devices use 5 wires, and sometimes 6 wires. You can always know that these are wide band sensors. There are no 5 or 6 wire narrow band sensors. Case closed.
  3. Check the manual for your vehicle. If you're going to be installing modifications to your engine, you should have a Haynes or a Chilton's manual for your car or truck, preferably Haynes as I've found these generally more informative. If you don't want to buy one, you may be able to find them at your local library. You can also google for the manual for your vehicle. I was able to download the entire manual for my Suzuki direct from Suzuki. It included all the wiring diagrams.
  4. Another alternative is to get your wiring diagram. You really want to have this anyway because it makes the EFIE install much easier. Here is an article that can help you with that: Wiring Diagrams. If you have a wide band sensor you will see it noted as such in the diagram. Otherwise it will be called, "Oxygen Sensor" or "Heated Oxygen Sensor" or sometimes HEGO (Heated Exhaust Gas Oxygen) sensor.
  5. Use the chart of Color Codes. This small chart covers the vast majority of sensors in use. But note, these are the wire colors on the sensor itself. Once the sensor plugs into the vehicle's wiring harness, the colors change. So, this chart is for the first 6" of wire coming off the sensor. As a note about the chart: Sensors with a blue wire have always been found to be 4-wire wide band sensors used by Japanese car makers. That may not be universal, but has always been the case when I've seen them. The other sensors in the chart are all narrow band. You will also note that in all cases, the heater wires are the same color, and are white or black. These little details can help you isolate the signal wire.
  6. You can test your wires to see what kind of sensors you have. Use the Installation Instructions and read through section "1. Locate the oxygen sensor signal wire". This will describe how to determine which wires have which function from your oxygen sensor. If you have a narrow band sensor, you will find a signal wire that behaves as described in the instructions. If you have an AFR sensor, you will get different electrical phenomena. But in both cases, you can determine which one is the signal wire. Note, that in almost all cases, the 2 heater wires are the same color. Usually both white or both black. That narrows it down quite a bit.

If you are going to install an EFIE, it is a good idea to get the wiring diagram for your vehicle even if you are able to determine your sensor type by other means. It is much easier to access the wiring near the computer, than it is to try to attach wires at all of your sensor locations. This is covered more thoroughly in the EFIE instructions. But, in order to do this, you need to know the wire colors and pin numbers where the wires go into the computer. Since there can be 50 or more wires, you need the diagram if you want to install near the computer. I always get one on my installs by whatever means necessary.


I hope this page hasn't confused you. We have quite a few years of experience dealing with these sensors, and I wanted to be able to give you as much information as I could to help you deal with yours. This is not intended to be an exhaustive description of how every type of oxygen sensor works. It is really a broad strokes description of the 2 main types of sensors in use in fuel injected vehicles today, and how to figure out which type you have. Let me know if you run into any questions.

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