Throttle Position Sensor (TPS) – Function – Failure – Testing

Throttle Position Sensor (TPS)
Throttle Position Sensor (TPS)

So your car was running fine, but suddenly it’s, stalling, jerking, idling poorly, surging, and just generally running poorly.

Any, Or all of these systems could point to a problem with your throttle position sensor (TPS).

The function of the throttle position sensor (TPS) is to supply information to the (ECU) about the position of the engines throttle.

The (TPS) converts the (throttle valve) position into a proportional voltage signal which is fed to the engine control unit (ECU).

As a result, The information from the throttle position sensor (TPS) is critical for proper start up and smooth throttle response.

The (ECU) uses this and other information to calculate the proper amount of fuel injection timing, ignition timing, camshaft timing, anti-lock braking system and transmission shift points.

Throttle Position Sensor (TPS) Types

There are only two types of throttle position sensors:

Switch Type

  • In a switch-based (TPS), the switch is always on. As a result, Providing a continuous current of electricity while the throttle is in use. When the throttle is not in use, the switch is off and does not allow electricity to flow.

Potentiometer Type

  • A potentiometer, however, sends a very low voltage to the (ECU) when the ignition is on but the throttle is off. It then increases the voltage as the throttle is increased, usually reaching a peak of 5 Volts when the throttle reaches its maximum.

Throttle Position Sensor- What Does It Do

Throttle Position Sensor (TPS)
Throttle Position Sensor (TPS)

The (TPS) reports the position of the throttle valve to your engine’s computer. The (TPS) also relays information to your anti-lock braking system so that it can supply the traction aids your car needs at a given speed. Most engines use only one (TPS), but more complex engines may have multiple sensors.

The engine’s computer starts by sending a set amount of voltage to the (TPS). There, the (TPS) applies a certain amount of resistance, depending on how open the throttle valve is, and sends the altered voltage back to the (ECU). As the throttle valve is opened and closed, that voltage changes.

The (ECU) uses this and other information to calculate:

  • Fuel injection timing
  • Ignition timing
  • Camshaft timing
  • Anti-lock braking
  • Transmission shift points

Since the (TPS) contains both electrical and mechanical parts, the throttle position sensor can be prone to failure over time. Sending voltage readings to the (ECM) that are out of spec, possibly illuminating the CEL and setting codes.

Warning Signs-Failure Symptoms

  • Delayed reaction from when you press the gas pedal
  • Jerks and bucks while the engine is under moderate load
  • Surges in idle speed
  • Engine stalls without warning and for no apparent reason
  • Poor power, poor fuel economy
  • Hesitation while accelerating
  • Sudden surges in speed on the highway
  • Delayed shifts or difficulty in changing gears (information from the TPS informs the ECM’s shift points for the transmission)
  • Illuminated or flashing (CEL)

In some cases, these symptoms may come and go before the sensor fails completely. It may require a few bad readings before the (ECM) will store a trouble code and illuminate the (CEL).

TPS sensor and circuit related DTCs:

  • P0120 Throttle Position Sensor/Switch A Circuit Malfunction
  • The most common code is P0121 is for “TPS “A” Circuit Range Performance Problem”.
  • P0122 Throttle Position Sensor/Switch A Circuit Low Input
  • P0123 Throttle Position Sensor/Switch A Circuit High Input
  • P0124 Throttle Position Sensor/Switch A Circuit Intermittent

Basic Testing

In some instances, problems with a (TPS) might originate from a bad electrical connection or corroded contacts on the connector. Be sure to check this before going any farther.

Use your multimeter for diagnosis:

  • Slowly open the throttle and observe the voltmeter.
  • The voltmeter reading should increase smoothly and gradually.


(TPS) Sensor
(TPS) Sensor

Problems from the throttle position sensor (TPS) often trigger a diagnostic fault code. Before replacing the sensor, the sensor’s wiring should be inspected for damage or loose connections. The sensor may also become loose causing the readings to be inaccurate.

The powertrain control module (PCM) supplies a 5 Volt reference signal to the (TPS) and usually a ground. A general measurement is: at idle = .5 Volts; full throttle = 4.5 Volts. If the (PCM) detects that the throttle angle is more or less than it should be for a specific (RPM), it will set a code.

Also, If you have no (TPS) signal, check for 5 Volt reference at the connector. If it’s present, check the ground circuit for open or shorts. Make sure the signal circuit isn’t 12V. It should never have battery voltage. If it does, trace circuit for short to voltage and repair. Look for any water in the connector and replace (TPS) as necessary.


A bad (TPS) can cause inability to shift up, rough or slow idle and lacking power when accelerating.

Limp Mode – Reduced Engine Power

Engine Power Is Reduced
Engine Power Is Reduced

So, The computer receives a signal from the (TPS) stating that the pedal is all the way to the floor. But, The throttle is actually closed. It would spot this error when it compared this status with the vehicle speed sensor. But, Would be signaling a low or no speed. As soon as it sees this discrepancy, the computer will command the transmission to go into limp mode.

Limp mode is a security function for your engine and transmission. When the engine or transmission control unit has recognized a serious faulty value from the engine or transmission, the car will go into limp mode. The limp mode does often reduce the power and limit the RPM of the engine so you can drive your car to a workshop without damaging the engine.

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