It will be either the cable or the pedal this is based on my 500 and some help from Brian Rylance.
The cable needs disconnecting at either end and pulling through - you can then decide whether its worth keeping or needs junking. Check the condition of the cable cover at the rear end - it might be split or kinked. Squirt a little oil in the cable cover at the front end.
The pedal...
Remove the seat (its just easier), pull the carpet/mat out the way and and unbolt the pedal from the from the floor. See if it spins freely in its outer tube.
As its out I'd clean it up anyway - remove the clip that holds it in position at the cable end, spray both ends with WD40 and then slide the pedal rod up and down its shaft, cleaning up what you can see with emery cloth and wd40. It might be really stiff and rusty, in which case you can drill a couple of 2mm holes in the outer shaft about 1 inch from each end and keep spraying in some 3in1 or wd40 and flushing out any rust. There are nylon bushes at the ends of the shaft which need to be kept intact so don't drill through them or put anything in the shaft that will degrade them. Keep moving the shaft up, down around, cleaning up and using oil or wd40 until it seems restored and spins freely in the shaft. When all is well, grease each end and plug the holes with grease if you drilled them. Grease the floor bolts when you put it all back.