[NiftySVX]

It detects the change in temperature as the hot gas flows by it, this is how it knows the valve is open. If there is no temp increase when the valve is open or the temp isn't down when the valve is closed it thinks there is a problem with the valve. This is the point of the sensor. The code for the sensor is only set if there is an open or short in the circuit. Usually when There is an egr problem on a vehicle like this you would just replace the valve, but it is possible to clean it.

Edit:

I meant valve and sensor together. Your low idle could be caused by the egr being slightly stuck open. If the egr opens at idle it will stall the motor. A common test is to apply vacuum to the egr and see if it stalls while it is idling. If it does, you know the valve is operating.