I tried to post this as a response to an old thread about the Zoom G3X expression pedal not working, but was not able to post there.

So, just in case anyone else is experiencing this problem, I thought that I would post the solution.

And thanks to 6string, on the Zoom site, and another poster somewhere else, for significant help in solving this

The answer is sort of in the manual, but it is not at all clear, and misses out one step.

To (re-) associate the expression pedal with a function, one does the following:

1. Scroll to the effect that one wants to have the expression pedal control (e.g., wah)
2. Click the footswitch to turn the pedal 'On' (light glows red)
3. Press the 'Total' button, in the row of buttons running horizontally across the top of the G3X
4. Go to 'Page' option of Middle Effect - specific instructions are in the manual on pages 17 - 19
5. OPERATE the expression pedal while doing the above
6. Return to Patch

I cannot remember exactly what I did in step 4, in terms of changing the settings, but there are very few options/permutations, and - if you strum the guitar, and keep on operating the expression pedal with your foot while adjusting the parameters/options, you will hear the effect kick in at some point.

The key to this procedure is to:
1. Set the effect that you want to associate the pedal with to 'On'
2. Go to 'Total'
3. Operate the expression pedal while changing settings

This was really frustrating me, so I am glad to have this resolved.

I hope that this is helpful to others.