Hi All,

Went to get the Legacy (MY'93 UK Turbo) MOT'd today and it failed on emissions. Only mods are a H&S backbox (both cats still in place).

Have been told that a dodgy Lambda sensor might be the cause of the problem, but I haven't a clue what it does or how it effects the emissions of the car. Can anyone enlighten me?

 

Following extract taken from www.lambdasensor.com...

The Lambda Sensor is at the heart of emission control continually monitoring the exhaust gas. Carbon Monoxide (CO) is produced by the engine. If the mixture supplied is too rich then CO will be high and visa versa. Having plenty of fuel (Rich) the engine will try to burn as much as possible, using up all available Oxygen, conversely if there is not sufficient fuel to maintain a correct burn then the excess Oxygen will pass into the exhaust system.

Rich mixture = High CO = Low Oxygen
Weak mixture = Low CO = High Oxygen

The Zirconia lambda sensor, by using precious metals, can determine the Oxygen difference between atmosphere and the exhaust gasses. The greater this difference, the higher the voltage it will produce, up to app 1volt. Lambda sensors will not operate until around 300 deg. They are heated by the exhaust and often have their own heater element. Single & twin wire lambda sensors have no heater element and are usually situated near or in the manifold, three & four wire sensors have an built in heater to aid rapid warmup and may be placed further downstream.