DIY

Bad Throttle Position Sensor Symptoms: What You Need to Know

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Your car has dozens of sensors onboard—one of which is the throttle position sensor (TP). As you might have guessed, the TP sensor measures the position of the throttle blade.

Older vehicles have just one TP sensor, whereas newer models with electronic throttle control (ECT) have two sensors as a fail-safe measure.

Although the TP sensor is a relatively simple part, it can cause several noticeable symptoms if it fails.

throttle position sensor

Symptoms of a Bad Throttle Position Sensor

The TP sensor is a potentiometer—a device that changes resistance (and therefore, voltage) in response to an adjustment knob or terminal. In the case of the TP sensor, the resistance changes in response to the movement of the throttle shaft.

Typically, most single TP sensors have an output voltage range between 0.5 volts (throttle closed) and 4.5 volts (throttle wide open).

But in an ETC system that uses dual TP sensors, the output from one sensor is inversely proportional to the other. That means, one sensor will read 4.5 volts when the throttle is closed (and 0.5 volts when the throttle is open). The other sensor will read 0.5 volts when the throttle is closed (and 4.5 volts when the throttle is open).

Your car’s primary computer, which is also known as the powertrain control module (PCM), uses the TP sensor signal in a variety of ways. For example, the PCM uses the input from the sensor when calculating ignition timing, fuel delivery, and automatic transmission operation.

Common throttle position sensor symptoms include:

Illuminated Check Engine Light

The PCM monitors the TP sensor to ensure its working properly. If the module perceives a problem with the TP sensor or its circuit, it will turn on the check engine light.

Engine Performance Problems

As was mentioned, the TP sensor is an input to the PCM for fuel delivery, ignition timing, and other outputs. Therefore, problems with the TP sensor can easily lead to engine performance problems, such as:

Automatic Transmission Performance Issues

Because the TP sensor is an input to the control module for various automatic transmission functions (e.g., shift points and torque converter clutch operation), a problem with the sensor can cause transmission performance issues.

For example, you might experience harsh or delayed shifting.

automatic transmission
TP sensor issues can cause harsh or delayed shifting.

Vehicle Stuck in “Limp Mode”

Modern vehicles with ETC use a pair of TP sensors as one of the primary inputs for throttle control. If one of the sensors fails, the control module will likely put the vehicle in “limp mode” or “reduced power mode” as a fail-safe measure.

How to Test a Throttle Position Sensor

You can test a throttle position sensor with a digital multimeter (DMM), a scan tool, or an oscilloscope.

Using a DMM is the method most DIYers are comfortable with. The following steps will give you an idea of what the troubleshooting process typically involves.

Note: The following are general guidelines for educational and entertainment purposes only. Consult your vehicle’s factory information for specific repair instructions and recommended safety procedures.

Step 1. Check for Diagnostic Trouble Codes (DTCs)

Is there a diagnostic trouble code (DTC) for the TP sensor stored? Then there’s probably a problem with the TP sensor or its circuit.

In most cases, cars built after 1996 with onboard diagnostics (OBD) II will set a code for a bad TP sensor. Many older cars with OBD I will also set a DTC when there’s a problem with the TP sensor. You can check for diagnostic trouble codes using a scan tool or code reader.

But keep in mind: TP sensor codes don’t necessarily mean the sensor is bad. You’ll need to do some troubleshooting to rule out problems elsewhere in the circuit.

Step 2. Perform a Visual Inspection

If you find TP sensor-related codes stored, you can move on to performing a visual inspection. You’ll want to look for issues, such as damaged wires and poor connections.

Repair any problems found, then clear the DTCs, and see whether the problem returns.

Step 3. Test the TP Sensor Circuit with a DMM

Next, if you didn’t find anything obvious during the visual inspection, you’ll want to test the TP sensor with a DMM. In most cases, the TP sensor will have three wires going to it: reference voltage (from the PCM), return signal (from the sensor), and ground.

Here’s how to test the reference and ground portions of the circuit:

  1. Put on your safety glasses.

  2. Consult a repair manual to determine which terminals are which on the throttle position sensor connector.

  3. Test for power (reference voltage) going to the sensor
    -Remove the electrical connector from the TP sensor.
    -Turn the vehicle’s ignition switch ON.
    -Set your digital multimeter (DMM) to the DC volts setting.
    -Connect one meter lead to the battery’s negative terminal. Connect the other meter lead to the reference voltage terminal on the harness side of the TP sensor connector.
    -If you see around 5 volts, the reference voltage portion of the circuit is intact.

  4. Test the ground side of the circuit:
    -Make sure the vehicle’s ignition switch is turned OFF.
    -Set your digital multimeter (DMM) to the ohms setting.
    -Connect one meter lead to the battery’s negative terminal. Connect the other meter -lead to the ground terminal on the harness side of the motor connector.
    -A reading of anything other than out of limits (OL) indicates the ground side of the circuit is intact.
reference voltage
Checking for power (reference voltage).
ground check
Checking for ground.

If you find both sides of the circuit to be okay during testing, you can move on to checking the TP sensor itself. On the other hand, if your test results indicate a circuit problem, you’ll need to trace the factory wiring diagram and repair the issue as needed.

Step 4. Test the TP Sensor with a DMM

Note: This test will not work on a vehicle with electronic throttle control. It only applies to vehicles with a traditional, mechanical throttle body.

  1. Turn the vehicle’s ignition switch ON (but leave the engine off).

  2. Leave the electrical connector connected to the TP sensor. Use back probe test leads to connect the DMM between the TP sensor signal wire and ground wire.

  3. With the ignition on and throttle closed, you should get a reading of around 0.5 volts.

  4. Operate the throttle cam by hand and slowly open the throttle.
    -If the sensor is working properly, the voltage signal will increase as the throttle opens. -The increase in voltage should be smooth without any dropout spots.
    -Once the throttle is completely open, the reading should be around 4.5 volts.

  5. Slowly return the throttle to the closed position while looking for a corresponding decrease in voltage front the TP sensor.
    -A good sensor will produce a smooth, decrease in voltage without any dropout spots.
checking TP sensor signal
Checking the TP sensor signal with the throttle closed. As you can see, the value can vary; this good sensor reads around 0.7 volts.
opening throttle
The voltage reading should increase as the throttle is opened.

If the TP sensor does not respond correctly to the test mentioned above—and you’ve already ensured the circuit is intact during step #3—the sensor is likely faulty and needs to be replaced.

Before condemning the sensor, however, it’s wise to inspect the throttle body for binding and excessive carbon buildup. Issues, such as these can prevent the TP sensor from reading properly.

Here is an excellent video that outlines the TP sensor testing process:

You can also choose to remove the TP sensor from the vehicle and test it as outlined in this video:

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Author

Mia Bevacqua

Chief Mechanic at CarParts.com

Mia Bevacqua is an automotive expert with over 15 years of industry experience. She holds ASE Master, L1, L2, and L3 Advanced Level Specialist certification, as well as a bachelor's degree in Advanced Automotive Systems.

Throughout her career, Mia has applied her skills toward automotive failure analysis inspections, consulting, diagnostic software development, and of course, freelance writing. Today, she writes for companies around the world, with many well-known clients showcasing her work.

Mia has a passion for math, science, and technology that motivates her to stay on top of the latest industry trends, such as electric vehicles and autonomous systems. At the same time, she has a weakness for fixer-upper oddballs, such as her 1987 Chevy Cavalier Z-24 and 1998 Chevy Astro Van AWD.

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