There is a little electronic "wiper" that wipes across a piece of resistive material. It is basically a variable resistor. The resulting electrical signal is used to drive the throttle plate, I believe.

Well, a guy did a failure analysis on broken ETMs and found that the wiper was wearing a groove in the resistive material, until eventually it stopped working. Every time you step on the accelerator or change your position on the gas pedal, the wiper wears the resistive material more. City driving (more throttle changes / mile) = faster failure. Highway driving makes for slower failure, but it will still go.

When it fails, the car may suddenly (and without warning) just stop. Not good if you're on the highway or trying to pull out in front of a big truck. If you see the ETS light come on.... heed the warning and get it replaced, even though it will cost $1,000.

There is no preventive maintenance you can do. The wiper is completely sealed inside the throttle body unit.

Failure doesn't seem to have anything to do with cleaning the throttle body, which many people have tried with limited success.

http://www.volvoxc.com/forums/showthread.php?t=4875&page=10&pp=10&highlight=throttle

Good luck