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Citroën A-type Lambda Sensor info

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What is a Lambda Sensor?

A Lambda sensor is a device that measures the amount of oxygen in the exhaust gasses. Therefor it's also called an O2 sensor (O2 being oxygen).

How does a Lambda Sensor work?

Depending on how much oxygen is found in the exhaust gasses, the O2 sensor gives a voltage signal. This only works when the sensor is hot enough. Sensors are heated by the exhaust gasses passing by the sensor. Modern sensors have a internal heating element that makes sure the sensor is always kept at operating temperature. These sensors can not stand leaded fuel and silicone sealer (as used in some liquid gasket products). The sensor gives 0 volt when the engine runs lean and about 1 volt when the engine runs rich.

How do I fit a Lambda Sensor to my car?

A hole is drilled in the exhaust, as close as possible to the engine but preferably at a point where the gasses of all cylinders pass the sensor. Translating the voltage signal into something visible can be done by a voltage meter. However, analogue voltage meters can damage the sensor because they draw too much current from the sensor. Digital voltage meters are usually too slow to react to the signal changes. Therefor most of the time a specially designed meter is used. Designs for such a device can be found on the internet. The complete meter can also be bought from most specialists dealing with tuned cars and or EMS.

Which types of Lambda Sensor are there?

  • 1 wire
    The wire is the signal output. This sensor is unheated and will take longer to give a signal.
  • 2 wire
    This is basically a 1 wire sensor with separate ground wire.
  • 3 wire
    This is a heated sensor. 2 wires are for heating the sensor, usually they are the same color as well. They need a 12 volts feed and ground to get the sensor heated up. The 3rd wire is the signal output, usually a different color compared to the other 2.
  • 4 wire
    This is a heated sensor with additional ground. Usually the heating wires are white, the signal output wire is black and the ground/earth wire is grey.
  • 5 wire
    The only true wide band Lambda sensor!
For the first basic tuning a 1 or 2 wire sensor is good enough. Do remember that an unheated sensor will only work with a thoroughly warmed up car. Also, while idling or under low load conditions, the sensor will cool down and not give help full reading. A 3 or 4 wire heated sensor is needed to be able to measure in all conditions. This sensor still needs some time to heat up though.

Where can I get a Lambda Sensor?

Usually the easiest place is a scrap yard. Any modern fuel injected car has one.

What's the difference between wide band and narrow band?

Now it gets interesting. Normal lambda sensors are so called narrow band and have one major flaw. They only indicate that you are running rich or lean but they can't tell you how rich or how lean. This is because when you get outside the stoichiometric AFR (14.7 air to fuel ratio) the sensor output voltage rises or drops drmatically making it very difficult to see exactly where on the curve you are.

The solution. The wide band sensor. This is the 5 wire sensor, which has a built in oxygen pump and heater circuit to achieve a very low change in output voltage even outside the stoich area. So what's the catch? There is only ONE wide band sensor. It's made by NGK/NTK/BOSCH (all the same sensor) and can be found using the following info:

Application/part number: 92-95 Honda Civic, 1.5 VTEC, 36531-P07-003 (us version only apparently)
NAPA Part number OS791

For more info see:

A-type Tuning Index
Cats Citroën Net Index