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Lambda light

I just wanted to tell you guys, regarding the call about the lambda (?) light on the Volvo’s dashboard, that it comes from the “lambda factor”, which is the air-to-fuel weight ratio divided by the stoichiometrical reaction of the perfect mixture.

In other words, it is a factor that says how far is a mixture from being ideal. With optimal conditions you would have a lambda factor of 1. Anything below that value indicates a rich mixture and anything over it a lean one.

So, since the oxygen sensors used in cars determine the air-to-fuel ratio, they are also called lambda sensors or sonds.

Hope this is useful.

Cheers from Buenos Aires, Argentina.


I agree with santix but for the choice of words.

The “lambda” O2 sensor can not directly measure the O2 content of the exhaust. The sensor is designed to peak (or dip, depending on how its wired) when the stoichiometric mixture is detected. When the value rises (or dips) when mixture is adjusted then the correction is going in the right direction. Sometimes it has to be leaned, sometimes richened. It can not indicate lean or rich without comparing previous values. When values are plotted they resemble the greek symbol lambda. The stem resembles the sensor response while it warms up.

As to whether the lambda peak is the “perfect mixture”, I disagree. Mid-1990’s the EPA got upset when automakers quit blindly obeying the commands from the O2 sensor when they found they could run leaner under highway conditions, still meet the emission regulations, but get 25% better MPG. This change in software mostly negated the EPA’s oxygenated fuel edits.