The principal feature of this type of sensor is a finger-shaped ceramic element. It is provided with a separate heater to achieve the minimum operating temperature of 350 °C required for accurate control. On the exhaust gas end, the sensor housing is fitted with a protection tube to protect the sensor element against combustion residue in the exhaust gas.
The planar lambda sensor is a more advanced form of the finger-type sensor. The shape of the sensor element resembles an elongated plate. As both the measurement cell and the heater are integrated into this plate, the sensor is able to attain its operating state more quickly.
The planar sensor has a double-walled protection tube.