the three-way catalytic converter (TWC). This check is carried out when the conditions for the catalytic converter diagnostics have been met. The rear
heated oxygen sensor (HO2S) has no direct effect on regulation of the fuel/air mixture. However the Engine Control Module (ECM) uses the signal to
optimize the signal from the front heated oxygen sensor (HO2S). For more information, see: Three-way catalytic converter (TWC) diagnostics See:
Powertrain Management/Computers and Control Systems/Description and Operation/Three-Way Catalytic Converter (TWC) Diagnostics
The heated oxygen sensor (HO2S) uses voltage control. The signal characteristic is binary. With a binary signal characteristic, the amplitude of the
signal curve changes considerably when changing the oxygen content in the exhaust gases. Otherwise its components and function are the same as the
front heated oxygen sensor (HO2S).
Caution! The air lines for the heated oxygen sensors must not be trapped or damaged in any way. The connectors for the heated oxygen sensors
must not be greased under any circumstances. The oil in the grease would disrupt the reference air and the function of the heated oxygen
The engine control module (ECM) can diagnose the rear heated oxygen sensor. The signal can be read using VIDA.
Preheating of the heated oxygen sensors (HO2S)
The heated oxygen sensor (HO2S) only functions above a certain temperature, approximately 300 °C. The normal operating temperature is between
300-900 °C. The heated oxygen sensors (HO2S) are electrically pre-heated so that operating temperature is rapidly reached. They are also pre-heated to
ensure that the heated oxygen sensors (HO2S) maintain a normal operating temperature and to prevent condensation which could damage the heated
oxygen sensor (HO2S).
The heater element in the probe consists of a positive temperature coefficient (PTC) resistor. The system relay supplies the heater element with voltage.
The element is grounded in the engine control module (ECM). When the control module grounds the connection a current flows through the PTC
resistor. When the heated oxygen sensor (HO2S) is cold, the resistance in the PTC resistor is low and a large current will flow through the circuit. The
current from the Engine Control Module (ECM) is pulsed at first to prevent condensation damage to the heated oxygen sensor (HO2S). Depending on
the temperature, allowances are made for factors such as the dew point. As the temperature in the PTC resistor rises, the resistance rises, the current falls
and switches in stages to a constant current. The pre-heating time for the front heated oxygen sensor (HO2S) is short, approximately 20 seconds.
The heater element heats the heated oxygen sensors (HO2S) to approximately 350 °C. The probes maintain this as a minimum temperature.
The engine control module (ECM) can diagnose the heater element.
Engine coolant temperature (ECT) sensor
The engine coolant temperature (ECT) sensor checks the temperature of the engine coolant. The temperature of the engine coolant is required so that the
engine control module (ECM) can regulate:
the injection period
the idle speed
the engine cooling fan (FC)
the ignition advance
engagement and disengagement of the A/C compressor
The sensor is a negative temperature coefficient (NTC) type which is supplied with power from the control module (signal) and is grounded in the
The resistance in the sensor changes depending on the temperature of the coolant. Depending on the resistance in the sensor, a voltage (signal) is
transmitted to the Engine Control Module (ECM). The lower the temperature the higher the voltage (high resistance). A high temperature results in low
voltage (low resistance).
The engine coolant temperature (ECT) sensor is located beside the thermostat.
The engine control module (ECM) can diagnose the engine coolant temperature sensor. The sensor value can be read off using VIDA.
Engine cooling fan (FC) / engine cooling fan (FC) control module