Heated Oxygen Sensors (HO2S)
Heated Oxygen Sensors (HO2S)
the HO2S provide feedback signals to the ECM to enable closed loop control of the Air Fuel Ratio (AFT). Four HO2S are installed, one pre-catalyst and one post-catalyst per cylinder bank. Each HO2S produces an output voltage which is inversely proportional to the oxygen content of the exhaust gases.
Each HO2S 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 HO2S. Injector pulse width is the length of time the injector is energized 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.
HO2S Principle of Operation
the ECM continuously checks the signals supplied by the HO2S for plausibility. If it detects an implausible signal the ECM stores a relevant fault code and details of engine speed, engine load and the HO2S signal voltage. the ECM requires the HO2S signals to set most of its adaptions. Failure of an HO2S results in most of these adaptions resetting to their default values. This in turn, results in loss of engine refinement. the engine may exhibit poor idle characteristics and emit a strong smell of rotten eggs from the exhaust (caused by an increase in hydrogen sulphide).
the efficiency of the HO2S slowly deteriorates with use and must be periodically replaced (currently every 120,000 miles but refer to the maintenance schedules for the latest service replacement period). the ECM is able to detect this steady deterioration from the HO2S signals. If a sensor deteriorates beyond a predetermined threshold, the ECM stores a fault code and captures details of the engine speed, engine load and battery voltage.
the HO2S needs a high operating temperature to work effectively. To ensure a suitable operating temperature is reached as soon as possible, each sensor incorporates a heating element inside the ceramic tip. This element heats the HO2S to a temperature greater than 350°C. the heating rate (the speed at which the temperature rises) is carefully controlled by the ECM to prevent thermal shock to the ceramic material. the ECM supplies a Pulse Width Modulated (PWM) supply to the heater elements to control the rate at which the HO2S temperature is increased. the HO2S are heated during engine warm-up and again after a period of engine idle.
the ECM monitors the state of the heating elements by calculating the amount of current supplied to each sensor during operation. If the ECM identifies that the resistance of either heating element is too high or too low, 4 will store a fault code, the engine speed, coolant temperature and the battery voltage.
HO2S are very sensitive devices. They must be handled carefully at all times. Failure to handle correctly will result in a very short service life, or non-operation. HO2S are threads coated with an anti-seize compound prior to installation. Care should be taken to avoid getting this compound on the sensor tip. If the sensor needs to be removed and refitted, a small amount of anti-seize compound should be applied.