The throttle position sensor (TPS) is part of your vehicle's fuel management system, and helps ensure that the correct mixture of air and fuel is delivered to your engine. The TPS provides the most direct signal to the fuel injection system of what power demands are being made by the engine. The TPS signal is continually measured and combined many times per second with other data such as air temperature, engine RPM, air mass flow, and how quickly the throttle position changes. These data determine precisely how much fuel to inject into the engine at any given moment. If the throttle position sensor and its other sensor partners do their job correctly, your car accelerates, cruises, or coasts smoothly and efficiently, as you expect, while maintaining optimal fuel economy.

The throttle position sensor can fail in several ways, all of which result in poor fuel economy at best, and performance limitations that may create safety hazard for you and other motorists at worst. This sensor can fail gradually, or all at once. In most cases, the Check Engine Light is illuminated if a TPS failure is detected. Also, most manufacturers provide a “limp home” mode of operation with reduced power if a failure is detected. This is intended to at least allow a driver to get off a busy highway in a safer manner.

Once the TPS begins to fail, even partially, you’ll need to replace it right away. Replacing the TPS will include clearing relevant fault codes, and may require software re-programming of the new TPS module to match other engine management software. This is all best left to a professional mechanic who can provide you with a diagnosis, and then install the correct replacement part.

Here are some common symptoms of a bad or failing throttle position sensor to watch for:

1. Car won’t accelerate, lacks power when accelerating, or accelerates itself

It may feel as though the car simply doesn't accelerate as it should. It may accelerate smoothly, but lack power. On the flip side, it may be that your car suddenly speeds up while you’re driving, even if you haven’t pressed the accelerator. If these symptoms occur, there's a good chance you've got a problem with the TPS.

2. Engine won’t idle smoothly, idles too slowly, or stalls

If you start to experience engine misfires, stalling, or rough idling when the car is stopped, it can also be a warning sign of a failing TPS. You don't want to wait to get this checked out!

3. Car accelerates, but won’t exceed a relatively low speed, or shift up

This is another failure mode of the TPS, which indicates that it is falsely limiting the power being requested with your accelerator pedal foot. You may find that your car will accelerate, but not beyond a speed of 20 - 30 MPH. This symptom often goes hand in hand with loss-of-power behavior.

4. Check Engine Light comes on, accompanied by any of the above behaviors

The Check Engine Light may come on if you’re having issues with your TPS. This isn’t always the case though, so don’t wait for the Check Engine Light to illuminate before you get it checked out. If you have any of the above symptoms, have your vehicle checked for trouble codes to verify the source of the problem.

The throttle position sensor is key to getting the desired power and fuel efficiency from your vehicle in any driving situation. As the symptoms listed above make clear, failure of this component has serious safety implications, and must be checked out immediately by a qualified mechanic.

This article originally appeared on as Symptoms of a Bad or Failing Throttle Position Sensor and was authored by David Wooten.

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