The throttle position sensor monitors how far open the throttle valve (or blade) is open, which is determined by how far down the accelerator pedal has been pushed. The throttle position controls the amount of air that flows into an engine’s intake manifold; when it’s opened wide more air flows in; when it’s nearly closed, little air flows in. The position of the throttle and how quickly it’s opening or closing is transmitted to the engine control module, and that information is among the factors the computer uses to decide how much fuel is injected into the engine and the spark timing. The TPS is usually mounted on the throttle body (the housing that contains the throttle valve). When a TPS malfunctions it can trigger a “check engine” warning light or cause the engine to idle roughly, surge, hesitate or stall because the engine control module doesn’t know what the throttle position is and can’t correctly set the fuel mixture or ignition timing.