Electronic Engine Controls - System Operation And Component Description (G1550549): Description: Pre-Heating

The HO2S operate efficiently at temperatures above 300°C (572°F). The normal operating temperature is between 300°C and 850 °C (572°F and 1562°F). In order to reach the optimum operating temperature quickly, the HO2S is electrically pre-heated by the ECM. Pre-heating the sensors also prevents condensation which can damage the sensor.

Each HO2S contains a PTC (positive temperature coefficient) resistor which functions as the heating coil. The resistor is supplied with 12V current from the ECM relay in the BJB and the ECM provides the ground for the resistor. When the sensor PTC resistor coil is cold, resistance through the resistor will be low allowing a high current to pass, heating the coil. The ECM controls the current using a PWM ground. As the resistor coil heats up, the resistance through the coil increases, reducing the current flow. The ECM reduces the duty cycle of the PWM ground until a continuous ground is provided. The ECM uses a PWM ground to prevent damage to the sensor by thermal shock due to the sensor heating up too quickly.

The PTC resistor coil is heated following an engine start for approximately 20 seconds and also during low load conditions where the exhaust gas temperature is insufficient to maintain the optimum sensor temperature. The ECM can diagnose faults in the PTC resistor and can record DTC 's which can be retrieved using a Jaguar approved diagnostic system.