Checking the exhaust gas temperature sensor

In modern vehicle models, exhaust gas temperature sensors are installed at various points in the exhaust gas system in both diesel and petrol vehicles. The exhaust gas temperature sensor detects the temperature, for example, in front of the catalytic converter or diesel particulate filter and sends this as a voltage signal to the engine control unit. The engine control unit needs this information in order to control the air-fuel ratio or the particulate filter regeneration and thus effectively reduce emissions. In addition, high temperature sensors protect components in the area of the hot exhaust gas flow from critical overheating.

The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for suitably qualified personnel only.

Exhaust gas temperature sensor

Figure 1: Example of PTC / NTC temperature behavior

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 ).

Exhaust gas temperature sensor - Symptoms

The following effects can occur in the event of an exhaust gas temperature sensor failure:

Warning lamp for glow plug system flashes
Warning lamp for particulate filter lights up
Engine control lamp is on
Increase in exhaust emissions (CO, NOx and HC)
Poor driving behaviour because of shortened regeneration intervals of the particulate filter
Increased fuel consumption caused by longer particulate filter regeneration
Loss of performance caused by incorrect detection of the degree of saturation of the particulate filter

Checking the exhaust gas temperature sensor

System overview

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.

Testing with the diagnostic unit

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.

Error code

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 our case study the error codes 15709 and 5257 are displayed, codes which indicate a fault in exhaust gas temperature sensor 3.

Checking exhaust gas temperature sensor - reading out error code — Any errors that occur are stored in the error memory of the control unit.

Parameters

In this function, the current measured values of the sensor in the control unit can be displayed and evaluated.

Illustration shows a vehicle with three exhaust gas temperature sensors
Sensor 3 shows a value of minus 40 degrees, which may be due to an interruption in the electrical wiring or to a defect in the sensor.

Checking exhaust gas temperature sensor - current measured values of the sensor in the control unit are displayed. — Current measured values of the sensor in the control unit are displayed.

Circuit diagrams

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.

Checking the exhaust gas temperature sensor - the PIN assignment on the exhaust gas temperature sensor or the cable colours can be read in the circuit diagram and used for further tests. — The PIN assignment on the exhaust gas temperature sensor or the cable colours can be read in the circuit diagram and used for further tests.

As part of troubleshooting, a visual inspection of the components and peripherals in the engine compartment should first be carried out after the ECU diagnosis.
The sensor should be replaced if the connecting cable or sensor housing is damaged.
If the threads of the exhaust gas temperature sensors are coated, they must not be treated with an extra coating of hot thread paste.
Exhaust gas temperature sensors must be tightened to the specified tightening torque! We recommend a tightening torque of 40 - 45 Nm for HELLA exhaust gas temperature sensors.

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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