Throttle position sensor – operation and diagnostics


The throttle is the key element through which the driver controls the gasoline engine. It is a type of butterfly valve that regulates the air supply to the engine. In the classic system, the gas (accelerator) pedal was connected to the throttle by a cable. Adding gas led to the opening of the throttle, which caused air to flow into the engine. In a system with a carburetor, this resulted in a simultaneous increase in the amount of fuel going to the engine – the engine increased revs and the car accelerated.

With electronically controlled injection, it is the engine controller (ECU) that determines the appropriate fuel dose based on throttle opening, using other parameters as well, such as air flow, temperature or intake manifold pressure. In order for the controller to “know” that the throttle is open, a sensor (sensor) is needed to convert the degree of opening into a signal in voltage form. The voltage is then transmitted to the ECU, where it is converted into opening data.

Nowadays, the accelerator pedal is no longer mechanically connected to the throttle – information about its depressing goes to the controller also in the form of an electrical signal. An electric drive is responsible for the movement of the throttle. In such a throttle (which is part of a servo), a sensor is necessary to set it to the required position.

How does the throttle position sensor work?

The throttle position sensor (TPS) is most often realized in the form of a potentiometer located on the extension of the throttle axis. Opening the throttle causes the slider to move along the resistance raceway, changing the resistance ratio at the outputs. The first systems with a cable-operated throttle were often limited to a single potentiometer. Over time, to increase reliability, additional contacts were added to signal the extreme positions – fully open and closed throttle.

Nowadays, most often the position sensor is integrated into the electric throttle actuator. The resulting servo is fully responsible for opening the throttle therefore it is crucial for driving safety. For this reason, a multiplexed sensor (as a rule, two potentiometers) is usually used, whose signals can be compared by the controller. This makes the data easier to verify and the sensor malfunction theoretically easier to detect.

Symptoms of a faulty sensor

What the effect of part failure will be is easy to predict if we understand its role well. Opening or closing the throttle is a clear signal to the controller to increase or decrease fuel dosage. If the signal is incorrect, the controller may select the wrong doses. There are a number of symptoms that could indicate a problem with the sensor:

  • Uneven engine operation;
  • The engine shuts down while running at slow speed;
  • Difficulty in starting the engine;
  • The car reacts incorrectly to the addition of gas;
  • Increased fuel consumption.

Causes of defects

A typical sensor made as a potentiometer is a mechanical component that is subject to natural wear. The slider making cyclic movements along the thrust path leads to its rubbing out, which can result in an uncertain signal in these regions of the path. The range most susceptible to wear is the region corresponding to low rpm. It is in this position that the slider is located much of the time and where it usually begins its movement.

Another typical type of damage are those resulting from moisture and corrosion of the components of the subassembly and its leads. The component is mounted under the hood of a car where there are large temperature fluctuations, and splashing through water or oil is not unusual. There are also mechanical failures that can cause a malfunction either directly or indirectly, such as by unsealing the housing.

How to diagnose throttle position sensors?

Due to the multiplication of the potentiometer, the fault should be detected by the ECU and recorded in the form of an error code. If this is the case, you can immediately proceed to verify the subassembly. In other cases, it is more difficult, because you have to find the source of the fault. The symptoms are not peculiar and can also mean at least a faulty MAF flow meter or a clogged air filter.

Evaluating the throttle position sensor should start by locating it and determining whether the part housing shows signs of damage such as cracks or dents. Look out for dirty or oxidized contacts and signs of corrosion. If you suspect a faulty sensor, it is worth checking the wiring by checking the continuity of the circuit between the engine controller connector and the throttle connector.

Multimeter or oscilloscope?

If you do not have additional equipment, but you have service data such as a circuit diagram or the correct resistance, you can try to measure this parameter with a multimeter. Such a measurement, however, may be too slow to detect a typical path rubbing problem.

An effective way to verify the signal from position sensors is through oscilloscope measurement. In this way, manually adjusting the throttle, we can observe the change in tension – it should be even, without sudden jerks or jumps in the mileage. Oscilloscopes dedicated to the automotive industry such as the Scope DT offer tremendous diagnostic capabilities. Just select the throttle from the list of components, the device itself will set the appropriate parameters. All the user has to do is connect to the ground and output signal from the sensor. One button is all it takes to bring up on-screen help for testing this component.

A clever way to test

Those who do not feel confident with an oscilloscope but want to make such a measurement can use the QST-5 sensor tester for a simple and quick test. We can carry it out even if we do not have information on the wiring diagram. All you need to do is connect the three leads of the potentiometer to the three inputs of the tester in any way you like (you can learn about how to identify the different pins in the case of the throttle module here: https://youtu.be/i6-y91n8-uw ).

The tester itself detects the type of sensor (potentiometer) and its pinout. Then simply press OK to proceed with the test. The sensor will be powered from the tester and the screen will show the output value and a small waveform illustrating the output from the sensor. Despite its small size, the graph is displayed in such a way as to highlight any irregularities or signal spikes during the measurement. In this simple way, we will make sure that there is indeed wear on the sensor.

Throttles in diesel engines

Initially, compression-ignition engines did not have a throttle, since the operation of a diesel engine can be fully controlled by fuel dosage. If engines don’t need it, why is it installed so often these days? It turned out that the addition of a throttle by affecting the pressure in the intake system, allows better control of the EGR exhaust gas recirculation system. In addition, this element makes it easier to turn off the engine.

When testing diesel engine throttles, you can use a dedicated tester in the form of the DTA-1. The tester is connected directly to the throttle so we can check it by setting it in any position. Correct operation of the subassembly together with the sensor will be based on uniform movement throughout the operating range and not exceeding the allowable current measured by the tester.


Repairing the throttle position sensor involves replacing it. In older designs, in which the position sensor occurred separately, the cost of the part is often several tens of zlotys. Dealer offerings include parts for basically any car. Newer vehicles usually have an integrated throttle module, which in that case must be replaced in its entirety. A complete component usually costs several hundred zlotys, so it is worth knowing how to check it. It’s good to have tools such as QST-5 sensor tester oscilloscope in the workshop so that you don’t waste time swapping out working components.


Author: Product Engineer Piotr Libuszowski


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