The throttle position sensor (TPS) on a vehicle does just what the name suggests: it measures how far down the gas pedal is depressed. In the days before cars had computers there was no TPS. With the advent of automotive computerization, fuel injection and increased miles per gallon, the TPS has become a very necessary part of the engine. The measurement provided by the TPS is used by the computer to adjust many parameters of the engine, primarily the amount of fuel delivered.
A malfunctioning TPS will cause your engine to run poorly. Generally the engine will idle fine but will have difficulty accelerating. This is because the computer isn’t telling the engine to inject as much fuel as is required, while at the same time airflow is increasing. When this happens most vehicles have the malfunction indicator lamp (MIL) illuminate, but this is not always the case since it depends on the severity of the failure.
What It is and How It Fails
The TPS is essentially a potentiometer in a plastic case and it fails due to poor contact of the wiper. This can cause erratic readings or no readings. It is possible that the vehicle may not accelerate and then suddenly accelerate, indicating erratic contact. Most vehicles have a limp home mode when a problem is detected with a sensor, allowing an average amount of fuel to be delivered to the engine. This allows a driver to continue driving the vehicle with decreased performance and fuel economy, but is only intended to get the vehicle to a place to be repaired.
How to Test Your TPS
Some tests can confirm a bad TPS. If the MIL is illuminated then the vehicle's trouble code should be read. Many auto parts stores will read the code for free and basic code readers can also be purchased for around $20. Devices that link to a phone to read the code can be purchased for as little as $5. Common fault codes related to a bad TPS are P0121 and P0122, but may vary by vehicle.
Finding the TPS for More Tests
To further test a TPS, it will need to located under the hood of the car and some measurements will need to be taken with 
CC BY-SA 3.0 Sfosketta multimeter. First, locate the throttle body under the hood. Consulting a repair book should help identify engine components. The throttle body is typically located near the air intake and has a butterfly valve to control air flow. Unfortunately, this test will be difficult, if not impossible on cars that use a drive-by-wire throttle system. A drive -by-wire system is where the throttle is controlled by a servo motor instead of a mechanical control cable.
The Voltage Test
Once the TPS is located, look for the three wires going to it. One wire is ground, one is the power--typically 5 volts and the third is the signal out. Identify the wires by using multimeter to measure the voltage on each wire. This test is done with the engine off but with the ignition in the on position. Once the ground and signal wires are identified measure the voltage output with the meter as the throttle is moved. On most engines the output will vary from about 1 to 5 volts. Look to see if the output changes smoothly as the throttle is adjusted up and down. Any sudden jumps or voltages outside of the expected range will indicate a bad TPS.
Another Way to Test it
As the TPS is just a potentiometer, an alternative way to check it is to directly measure the resistance with the multimeter. It is better to measure the voltage as previously described as it is a more complete test, but the resistance check is good to check a new or used TPS before installing it. The resistance check can be done with the ignition off or with the TPS removed from the vehicle. First, identify which is the contact to the wiper contact of the three pins. The resistance between the wiper and one of the two remaining pins will increase as the throttle is increased. The resistance between the wiper and the other pin will decrease as the throttle is increased.
The resistance between the two contac
ts that are not the wiper will remain constant no matter the position of the throttle. As with the voltage measurement look for a constant change or erratic changes. The actual resistance of the TPS will vary by manufacturer so the most important thing is to look for erratic changes as opposed to the values.
Replacing a TPS
Once a malfunctioning TPS has been identified it can be replaced with a new one. This is usually as simple as unplugging the old one, removing a couple of screws and installing the new one in reverse.
If the MIL light was illuminated it will need to be reset. This can be done with a code reader that has a reset button or by disconnecting the battery for a short time. In some vehicles, the new TPS sensor may provide slightly different readings to the computer than the old one. In this case it would require calibration by a repair facility although this isn’t usually necessary.
