Using a scan tool to check the Throttle Position Sensor

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How to Check the Throttle Position Sensor with a Scan Tool?

Throttle Position Sensor

A good scan tool can help you test the throttle position sensor of your car. That’s very important because the throttle position sensor (TPS) is an integral part of the car’s fuel management system. If it fails, the car will lack power and might turn itself off.

What is a Throttle Position Sensor?

A throttle position sensor is a type of sensor that monitors the air intake of a vehicle’s engine. It’s usually mounted on the butterfly spindle (also known as butterfly shaft) of the throttle body.

From there, the sensor can directly and accurately monitor the position of the throttle. 

The throttle body is the silver-colored component on which the throttle position sensor is attached. As you can see, there are cables running to and from the sensor to complete the sensor’s circuit. you can always diagnose both the sensor and its circuit with a scan tool.

What Does a Throttle Position Sensor Do?

As part of the fuel management system, the throttle position sensor helps to ensure that the correct mixture of fuel and air is delivered to the car’s engine. The engine needs those two to stay running.

It picks up air from the intake, passes it through the mass airflow sensor and into the throttle body. While there, the TPS will continually measure the amount of air. The engine then uses that data, alongside other values like RPM, air temp., and mass air flow in determining how much fuel to inject.

If the throttle position sensor is working properly, the car will move, coast, cruise, or accelerate smoothly. It will also run efficiently and maintain optimum fuel economy. You will notice this when you pull live data using a scan tool.

Now, with that said, exactly how does a throttle position work? It basically regulates how open the throttle valve is. That, in turn, is determined by how far down you have pushed the accelerator pedal. The valve will be fully open when the pedal is on the floor. Conversely, it will be almost fully closed when you completely release the accelerator.

As you manipulate the accelerator, the throttle position controls the amount of air flowing into the engine intake manifold. The TPS picks that information and relays it to the car’s engine control unit (ECU).

The ECU uses the information to determine how much fuel to inject. As mentioned already, if the throttle position sensor is working properly, the engine will inject the optimum amount of fuel for the idea fuel/air mixture.

If the TPS malfunctions, the ECU won’t know the exact throttle position and it my set an incorrect fuel/air mixture. That always results in poor fuel economy and many other issues that you absolutely want to avoid. Read the next section to find out more about a malfunctioned throttle position sensor.

What Happens when Throttle Position Sensor is Bad?

At best your car will have poor fuel economy and at worst you may get into an accident. The TPS is so important that if it fails, your car turns into a safety hazard. The throttle body won’t function as it should and if the car doesn’t automatically shut down, it will fail to change gears or set base ignition timing.

So, can you drive with a bad throttle position sensor? The car might be able to move, but you should never drive it if the TPS has failed. Use a scan tool to diagnose the issue and then fix it before attempting to take the car on the road.

Just in case you’re wondering, a failed TPS will force the throttle body valve to either shut or stick in an open position. In the latter case, the engine will receive excess air. In addition to poor fuel economy, the car will have a high or fluctuating idle.

If the valve remains shut, the car won’t even start. If by chance it does, it can automatically turn off possibly when you’re driving. Worse yet, the bad TPS may cause problems in other engine components, causing the entire engine to fail.

Keep in mind that TPS failure can happen slowly and gradually or abruptly.

Signs of a Bad Throttle Position Sensor

So, a bad TPS is not good news. But how do you know if the throttle position sensor has failed? A Good OBD scan tool should tell you that. However, below are some observable signs and symptoms of a bad throttle position sensor:

  • The engine will either idle too slow or stall completely. It won’t idle smoothly
  • The car will have erratic acceleration. Either it will accelerate by itself or it won’t accelerate even when you push the accelerator
  • Evident lack of power despite acceleration
  • Gear may fail to shift
  • Check Engine Light (CEL) may come on
  • Reduced fuel economy

A combination of two or more of the signs suggests that you have a real issue with the TPS. So how do you diagnose a bad throttle position sensor? Check out the throttle position sensor test below.

How to Check the Throttle Position Sensor with a Scan Tool

Will a bad throttle position sensor throw a code? Yes, it will. All the generic codes that are related to the TPS range from P0120 to P0124.

The most common throttle position sensor code is the P0122 – Throttle Position Sensor/Switch A Circuit Low Input. It’s triggered when the ECU detects that the TPS circuit A is outputting a lower voltage than expected.

The other TPS codes are:

  • P0120 - Throttle Position Sensor/Switch A Circuit Malfunction
  • P0121 - Throttle Position Sensor/Switch A Circuit Range/Performance Problem
  • P0123 Throttle Position Sensor/Switch A Circuit High Input
  • P0124 Throttle Position Sensor/Switch A Circuit Intermittent

If you connect a scan tool and get any of those codes, it means that there’s a fault in the TPS and/or the TPS circuit. You will need to isolate the problem. Here are the steps for doing that:

Step 1. Pull trouble codes

Using a scan tool, read all the trouble codes present in the car’s ECU memory. Make sure the vehicle is in ignition key on, engine off (KOEO). If you see any throttle position sensor code, proceed to the next step. It will almost always come with the Check Engine Light (CEL).

Step 2. Clear the codes

Erase all the codes. All the good scan tools should have that function.

Step 3. Perform driving cycle

Unplug the OBD scan tool and start the vehicle’s engine. In case the Check Engine Light turns off, then it was an intermittent problem possibly caused by changes in temperature. You have nothing to worry about.

If the CEL comes on again, do a 5 to 10-minute drive to see if it will turn off. If it doesn’t, read codes again to confirm that the TPS codes are still there. The presence of any one of them should prompt you to move to the next step.

Step 4. Check live data

Connect the scan tool again and switch the car to KOEO mode. On the scanner, navigate to live data and lock the TP sensor on the display screen. Use live data graphing if your scanner supports it.

Step 6. Analyze graph

Slowly push the accelerator pedal as you observe the live data. The graph should be a straight line with a positive slope. If it (the line) changes abruptly as you press the pedal, that means the TPS is faulty. The sudden change may either be a positive or negative slope. Either way, it indicates that your throttle position sensor is bad.

Fixing a Bad Throttle Position Sensor

If you’ve confirmed that indeed the TPS is faulty, you can go ahead and fix it. That basically means replacing it. You may need to relearn the new TPS with the ECU. That’s where scan tools with programing and coding come in handy.

Most car owners often wonder: can you clean the throttle position sensor? The answer is a resounding no. While it’s okay to clean the throttle body using a clean cloth and carburetor fluid, you should never attempt to clean the TPS. You can easily contaminate it or damage some of its wires. If that happens, the only fix is replacing the entire sensor.

Warning: always seek professional help when you’re not confident about your DIY repair skills.

Is it expensive to fix a throttle position sensor?

That depends on the make and model of your vehicle. The throttle position sensor location on some cars is usually out in the open. Most mechanics will charge you under €500 to replace those. That’s inclusive of parts and labor.

If the sensor is located deep in the engine where it’s hard to access, you may have to pay up to €1000 for the fix. In that case the labor cost will be higher because of the amount of work required. The mechanic may have to disassemble other parts like the intake manifold, throttle body etc. just to get to the TPS. It rather justifies the steep cost.


Justin Kavanagh
Justin Kavanagh is a recognised leader in automotive intelligence and vehicle data supply to the entire motor industry. He has almost 20 years experience in building systems from the ground up. As the Managing Director of Vehicle Management System, he understands the need and importance of trustworthy and reliable vehicle history and advice to both the trade and the public.
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