Dave Plowman (News) explained on 15/08/2016 :
Can one of our teccies explain how this is achieved in practice?
My last car had a throttle cable - although newer versions of the same
model went over to drive by wire. So it has been around for a long time -
15 years or more.

The idle is entirely separate / independant of the fly by wire sytem,
though both are controlled by the engine ECU. The throttle
potentiometer just signals the ECU to indicate how much speed/ power is
required. When at rest, the idle routine in the ECU takes over to hold
the engine speed precisely at the correct idle speed.

Which means your throttle sensor might not be the cause of the problem,
there are numerous other possibilities.