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What Do I Need To Know About My Oxygen Sensors?

Revised 30 Apr 09
To include data on
Downstream Sensors

Which EFIE Do I Need?

The first thing you have to know is that we have made this problem really simple.  There are now only 2 types of EFIE we recommend.  One type is for narrow band sensors, and one is for wide band sensors.  We will describe these more thoroughly below, but you should know that there are only 2 types.  We now produce a Dual and a Quad version of each type.  The Dual version is for vehicles with 2 sensors and the Quad version handles 4 sensors.  So there are only 2 types, and 2 versions of each type.

So, which EFIE do I need?  This question comes up a lot. There are literally thousands of models of car, and each has it's own design. The number of oxygen sensors can vary from one to four or more. Do all of them need EFIEs to operate properly with fuel saving devices? The short answer is, if you have electronic fuel injection, and oxygen sensors, then yes.  This applies to virtually all gas cars and trucks on the road today.  If you have a diesel engine, you don't need an EFIE (although modern diesels usually need a MAP/MAF Enhancer). 

Six and eight cylinder engines tend to have 2 sensors upstream of the catalytic converter, one on each exhaust manifold, but sometimes have only one. Four cylinder engines usually have 1 upstream sensor.  The same number of downstream sensors as upstream sensors are usually present, but sometimes there is only one downstream sensor.

The sensors after the catalytic converter, or attached to it, were originally designed to tell the engine computer when the catalytic converter has gone bad, but were generally not used to modify the calculations on how much gas to give to the engine. They left this job to the upstream sensors. However, this is no longer the case in most vehicles, and EFIEs on the downstream sensors are now needed to get them to respond. In some cases the vehicle's documentation reports this use of the downstream sensors, but in many cases there is no mention, even though treating the downstream sensors has solved numerous fuel saving projects. We have for some years now manufactured our main product line to accommodate both upstream and downstream sensors in one economical package.

What Type Of Sensor Do I Have?

It used to be that there were only narrow band sensors. However in recent years we have a new type of sensor, known as a wide band sensor, or A/F sensor (air/fuel sensor). You can find out more about this type of sensor in Oxygen Sensors, Types of. One type of wide band oxygen sensor uses 4 wires. These are usually found on Toyotas, but I expect to see these used more on other vehicles too. Most wide band sensors use 5 wires or even 6 wires.

How do you know if you have an wide band sensor or narrow band oxygen sensor? Well, it might be written on the vehicle information tag under your hood. Open the hood and look up. Otherwise, another way to find out is through documentation for your vehicle. I don't mean the owner's guide that is given to you when you buy your new car. 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 these are generally more informative.

Another alternative is to get your wiring schematics from Mitchell. The procedure for that is described in this article. If you have a wide band (also known as "AFR" sensor, or A/F sensor), you will see it in the diagram. Otherwise it will be called, "Oxygen Sensor" or "Heated Oxygen Sensor" or sometimes HEGO (Heated Exhaust Gas Oxygen) sensor. If you can wait a day or two, we will look your vehicle up for you and tell you what type, and how many sensors you have. Just fill out a Sensor Information Request Form, which gives us the data we need to look up your specific vehicle.

The most easily identifiable form of wide band sensor is the 5-wire wide band. As you might guess, these devices use 5 wires, and sometimes 6 wires, but the 6 wire version is the same sensor with an extra ground wire. You can always know that these are wide band sensors. There are no 5 or 6 wire narrow band sensors.

If you read the Installation Instructions, you'll see there's another reason to have a good wiring diagram for your car. That is so you can find the signal wires for your sensor(s) up near the computer, where you can easily access them. Believe me, they are worth the money for that alone. But if you have any doubt about the type of sensor you have, they are doubly valuable.

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 entirely.


I hope this page hasn't confused you. But it's best to start digging in and finding out what you really have under the hood, so you don't end up purchasing an EFIE that you don't need or can't use. If you still have questions or are unable to find the information on your sensors, fill out the Sensor Request Form, and submit it. We will get back to you with the number and type of oxygen sensors you have, and give you a recommendation on which EFIE you need.

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