PURE WILD ASS SPECULATION:
Most likely internal to the ECU, but if the engine's runing, maybe not.
If it's a throttle thing, it could be linkage, or.......it could be the throttle-by-wire on the engine. Remember that the throttle cable actuates the throttle position sensor, which in turn outputs a DC voltage varying between like 0 and 5 volts corresponding to sensed throttle position. That voltage goes to the ECU. This tells the ECU what the driver's asking for.
Once the ECU determines how best to give what the driver wants, it sends a signal to the throttle actuator to spin the butterfly, etc. in the intake. The actuator is right next to the position sensor. There's also probably some feedback from the actuator to the ECU to tell the ECU how far open the butterfly is.
Anyhoo, you can see that if the engine truly is running, there could be a problem with the throttle position sensor, the wiring between it and the ECU, between the ECU and the actuator, or in the feedback from the actuator back to the ECU. There could also be a problem in the throttle position sensor or the actuator.
I'd assume that if the ECU sensed a problem with any of that, it'd probably default to an idle setting.
All that being said, it's probably some lame vaccum line or something ;-)