Even though today's vehicles are fuel injected, they still have a throttle plate that is controlled by your right foot. That's what controls how much air flows into the engine's intake manifold. As you push harder on the accelerator, the throttle plate opens further. The TPS is attached to that throttle plate and its electrical resistance goes up as a contact within the sensor slides over an increased length of wire.

Using three wires (power, ground and output) the vehicle's computer sends a voltage through that variable resistor and measures its output. Using that information (and a bunch of other stuff), the computer knows how heavy your right foot is and decides how much fuel to inject. For those used to carburetors, the TPShas the same basic function as the accelerator pump, adding some extra fuel to prevent bogging when the throttle is suddenly opened.

As you might guess that sensor can wear out (after all, it's a mechanical device in my book). Or, it can get corroded or dirty. Then what happens? The voltage it puts out either gets "stuck" or it gets jumpy and the voltage bounces up or down as you press the gas pedal. Any of this leaves the computer pretty confused. One symptom is engine hesitation when you try to accelerate. It will often (but not always) set the check engine light. Typical trouble codes are P0120 P0121, P0122 and P0123.

So - how do you tell if it's bad? Plug in AutoTap or any scannerand read the Throttle Position Sensor voltage with the key on, engine off. I suggest using the "Graph-It" feature of AutoTap to be sure you don't miss anything (graphing capability is essential in any scantool). Now, slowly press the accelerator to the floor. You should see a smooth line on your PC or Palm. It should start down around 0.5 volts and rise to around 4.5 volts (exact values depend on the vehicle). If the line is bouncing down to 0 or up to 5, the sensor's bad. If it's stuck at a constant value, the sensor's bad. You can also do the same by connecting an analog (cheap) voltmeter to the TPS, and watch the needle movement.

This is an easy sensor to replace. You'll find it attached to the side of the throttle body (conveniently located at the top of the engine!). There's a wiring harness and a couple screws to remove - that's it! Depending on car the position of the sensor is adjustable and it must be bolted in in the proper position to work properly. All that is needed is a voltmeter and repair manual with the proper procedure.