244000414 - LAMBDA SENSORS

LAMBDA SENSOR UPSTREAM OF THE CATALYZER (A)

The lambda sensor measures exhaust gas oxygen content: it is fitted on the exhaust pipe upstream of the catalytic converter.lambda = 1 Ideal mixture (stoichiometric) lambda greater than 1 Lean mixturelambda less than 1 Rich mixtureExcess air; CO level tends to be lowLack of air; CO level tends to be highResistance: 4.5 ± 0.5 Ohm at 20° CThe sensor output signal is sent to the control unit for retroactive correction (feedback) of the mixture concentration. When the sensor supplies a low signal (voltage lower than 200 mV), the control unit detects a lean mixture and increases injection time. Then when the sensor signal is high (voltage higher than 800 mV), the control unit detects a rich mixture and decreases injection time. This event sequence is repeated with a frequency in the order of tens of Hertz to ensure that the engine runs on a mixture that fluctuates constantly about a stioichiometric level. At temperatures lower than 300° C, the ceramic material is inactive and the sensor cannot therefore send reliable signals: the sensor is fitted with an electrical heater coil to ensure fast heat at start-up and maintain a minimum temperature.

LAMBDA SENSOR DOWNSTREAM OF THE CATALYZER (B)

The downstream sensor is identical to the upstream sensor.

Sensors can be quickly be put out of service by the presence of even small amounts of lead in the fuel.

Sensor heater resistances may be measured by disconnecting the connector and connecting an ohmmeter as shown in the figure.