Heated Oxygen Sensors (HO2S) The HOS provide feedback signals to the ECM to enable closed loop control of the Air Fuel Ratio (AFR. Four HOS are installed, one pre-catalyst and one post-catalyst per cylinder bank. Each HOS produces an output voltage which is inversely proportional to the oxygen content of the exhaust gases. HO2S Each HOS consists of a zirconium sensing element with a gas permeable ceramic coating on the outer surface. The outer surface of the sensing element is exposed to the exhaust gas and the inner surface is exposed to ambient air. The difference in the oxygen content of the two gases produces an electrical potential difference across the sensing element. The voltage produced depends on the differential between the two oxygen contents. When the AFR is Lambda 1 (i.e. stoichiometric AFR of 14.7:1 by mass) the voltage produced is approximately 450 mV. With a lean mixture of Lambda 1.2, the higher oxygen content of the exhaust gases results in a voltage of approximately 100 mV. With a rich mixture of Lambda 0.8, the lower oxygen content of the exhaust gases results in a voltage of approximately 900 mV. The ECM monitors the effect of altering the injector pulse widths using the information supplied by the two HOS. Injector pulse width is the length of time the injector is energised, which determines how much fuel is injected. The response time is such that under certain driving conditions, the ECM can assess individual cylinder contributions to the total exhaust emissions. This enables the ECM to adapt the fuelling strategy on a cylinder by cylinder basis, i.e. inject the precise amount of fuel required by each individual cylinder at any given time.