Exhaust gas temperature sensor
Useful knowledge and valuable tips on the subject of testing the exhaust gas temperature sensor, illustrated using a VW Passat 2.0 16 V TDI from 2015 as an example.
Important safety note
An electrical resistor is installed in the measuring tip of the exhaust gas temperature sensor, the value of which changes depending on the temperature. These resistors are called thermistors and are divided into two groups. Here a distinction is made between NTC thermistors and PTC thermistors, depending on their temperature behaviour (Figure 1).
NTC - resistor with negative temperature coefficient (the resistance decreases with increasing temperature)
PTC - resistor with positive temperature coefficient (the resistance increases with increasing temperature)
The exhaust gas temperature sensor (EGTS) is usually connected directly to the engine control unit (ECU). Together with another resistor (RUP) in the control unit, the sensor forms a series connection through which an electrical voltage is divided. The signal is measured at the voltage divider and converted into temperature information by an algorithm (Figure 2).
The following effects can occur in the event of an exhaust gas temperature sensor failure:
The following causes can be responsible for the failure of the exhaust gas temperature sensor:
The following information is illustrated using a VW Passat 2.0 16 V TDI from the year 2015 as an example. Depending on the vehicle features, up to four exhaust gas temperature sensors can be installed. Engine control monitors the temperature before the catalytic converter, turbocharger and also upstream and downstream of the diesel particulate filter.
In vehicles without a SCR (Selective Catalytic Reduction) catalytic converter, the exhaust gas temperature sensor behind the diesel particulate filter is not required. In the event of a defective exhaust gas temperature sensor or in the case of implausible output values, the system switches to emergency operation or alternatively uses a substitute value to protect the relevant components from overheating. An error entry is then recorded in the engine control unit and the warning lamp in the dashboard is activated.
The function of the exhaust gas temperature sensor is monitored by the respective higher-level system control unit. Any errors that occur are stored in the error memory of the control unit and can be replaced by using a suitable diagnostic unit. Depending on the system, additional parameters can be displayed and used for troubleshooting.
In this function the error codes stored in the error memory can be read out and deleted. In addition, information on the error code can be called up.
In this function, the current measured values of the sensor in the control unit can be displayed and evaluated.
System-specific circuit diagrams can be taken from vehicle information and used for troubleshooting purposes. Here the PIN assignment on the exhaust gas temperature sensor or the cable colours can be read off and used for further tests.
Please also observe the respective maintenance and repair instructions provided by each individual vehicle manufacturer!
The various diagnostic options have been illustrated using the mega macs 77 diagnostic unit as an example. The respective test depth and variety of functions can be set out differently depending on the vehicle manufacturer and is dependent on the relevant system configuration of the control unit.
Schematic illustrations, pictures and descriptions serve to explain and illustrate the document text and cannot be used as a basis for vehicle-specific repairs.
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