Car Mechanics (UK)
Invented by Bosch and fitted first to the Federal-specification Volvo 200-series in the mid-1970s, Lambda sensors can now be found on many modern vehicles. They are critical to catalytic converter longevity and effectiveness, because the converter works only in very narrow air/fuel mix ratios, the optimum of which is known as ‘Lambda 1’ or 14:1. Lambda/oxygen sensors detect the exhaust gases’ oxygen content, prompting the engine ECU to adjust the fuel/air mixture accordingly.
As Lambda ratio is considered during an MOT test, note that the value is calculated. Any leak in the exhaust system might cause the Lambda value to be influenced sufficiently to cause an MOT fail. The sensors tend to be reliable, although they fail with time and can be ‘poisoned’ by exposure to glycol – be wary of Lambda sensor damage if your engine has suffered from head gasket failure, for example.
Take care when fitting a new Lambda sensor. Not only are they fragile, but also their plugs and cables should be kept away from heat sources. ELTA Automotive recommends that new sensors are fitted whenever you fit a replacement exhaust system/catalytic converter, not only because the existing items can be damaged easily, but because they also have a finite life of between about 45,000 miles for an unheated sensor and 100,000 miles for a heated type.
Once an old sensor is unscrewed (noting that you can buy dedicated Lambda sensor sockets from suppliers such as Laser and Draper) and you are not planning to replace the pipe, clean the threads with a suitable tap to reduce the risk of overtightening the new part, which risks cracking the sensor’s internal ceramic wall (on heated designs). Most new Lambda sensor threads are pre-greased and applying copper grease is not recommended, because you risk contaminating the sensor nose and under-tightening the part as a consequence. Should more than one Lambda sensor be fitted, replace them all together.