ENGINE MANAGEMENT SYSTEM – V 18-2-26 DESCRIPTION AND OPERATION HO2S Output A = Output, mV; B = AFR, lambda 1 Lambda window 2 Lean AFR 3 Rich AFR The ECM continuously checks the signals supplied by the HOS for plausibility. If it detects an implausible signal, the ECM stores a relevant fault code and details of engine speed, engine load and the HOS signal voltage. The ECM requires the HOS signals to set most of its adaptions. Failure of an HOS results inmost 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 HOS 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 HOS 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 HOS 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 HOS 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 HOS temperature is increased. The HOS 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, it 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. HOS are threads coated with an anti-seize compound prior to installation. Care should betaken 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 (see workshop manual for details).
ENGINE MANAGEMENT SYSTEM – V8 DESCRIPTION AND OPERATION 18-2-27