EMS Sensors (page 12 of 13)

ENGINE MANAGEMENT SYSTEM - SIEMENS

DESCRIPTION AND OPERATION 18-4-25

Sectioned View of HO2S

A = Ambient air; B = Exhaust gases 

1 Protective ceramic coating
2 Electrodes

3 Zirconium oxide

The material of the sensing element only becomes active at a temperature of approximately 300 

°

C (570 

°

F). To 

shorten the warm up time and minimise the emissions from a cold start and low load conditions, each HO2S contains 
a heating element powered by a supply from the main relay. The earth paths for the heating elements are controlled 
by the ECM. On start up, the current supplied to the heating elements is increased gradually to prevent sudden 
heating from damaging the ceramic coating. After the initial warm up period the ECM modulates the earth of the 
heating elements, from a map of engine speed against mass air flow, to maintain the HO2S at the optimum operating 
temperature.

The nominal resistance of the heating elements is 6 

Ω

 at 20

°

C (68

°

F).

If an HO2S fails, the ECM illuminates the MIL. If a front HO2S fails the ECM also adopts open loop fuelling and 
catalytic converter monitoring is disabled. If a rear HO2S fails, catalytic converter and front HO2S monitoring is 
disabled.

M19 2959

V

1

2

A

B

3