Engine management system - MEMS (V): Description and operation

The HO2S consists of a sensing element, the outer surface of which is exposed to exhaust gases, whilst the inner surface is exposed to ambient air. The sensor has a ceramic coating to protect the sensing element from contamination and heat damage.

V

3

1

A 2

B

M18 0605

A = Ambient Air; B = Exhaust Gases

1 Protective ceramic coating 2 Electrodes

3 Zirconium Oxide

CAUTION: HO2 sensors are easily damaged by dropping, excessive heat or contamination. Care must be taken not to damage the sensor housing or tip.

l The HO2S becomes very hot, take care when working near it. l Do not measure the resistance of the sensing element. l Observe the correct torque tightening value when installing the HO2S. l Do not subject the HO2S to mechanical shocks. l The HO2S may be contaminated if fuel with added lead is used.

The amount of oxygen in ambient air is constant at approximately 20%. The oxygen content of the exhaust gases varies with the AFR with a typical value for exhaust gas of around 2%.

The difference in oxygen content of the two gases produces an electrical potential difference across the sensing element. Rich mixtures, which burn almost all of the available oxygen, produce high sensor voltages. During lean running, there is an excess of oxygen in the mixture and some of this oxygen leaves the combustion chamber unburnt.

In these conditions, there is less difference between the oxygen content of the exhaust gas and the ambient air, and a low potential difference (voltage) is output by the HO2S. The ECM uses the voltage produced in the HO2S sensing element to calculate the AFR and thereby control fuelling to a high degree of accuracy.

The material used in the sensing element only becomes active at a temperature of 300°C (572°F), therefore it is necessary to provide additional heating via an electrical resistive element. The element uses a 12V supply from the main relay when the ECM energises the relay coil and allows a short warm up time and minimises emissions from start-up. The resistance of the heating element can be measured using a multimeter and should be 6Ω at 20°C (68°F).

a Rich AFR b Lean AFR

c Lambda window (0.97 to 1.03 mV) d HO2S Output in mV.