We, naturally, recommend Yamaha pedals. No need for a polarity switch as Yamaha pedals are built for Yamaha keyboards.
FC3A - is a sustain type pedal that is for serious piano work as it features half dampering. Sends 0 thru 127
FC4/FC5 - is a sustain type pedal and is your typical simple momentary On/Off Type, Sends 0 or 127

There is a Sustain Pedal parameter
Press [UTILITY]
Press [F4] CTL ASN (Controller Assign)
Press [SF2] FT SW (Foot Switch)
Sustain Pedal
Select FC3 if your pedal sends 0-127
Select FC4/5 if your pedal sends 0 or 127

See page 146 of the Reference Manual

Thanks for your reply. My MOX has always been switched to the FC3(half on) setting. I've tried it with the FC4/5 and it doesn't work either. It looks as though it's the sustain pedal itself. My owners manual doesn't have anywhere near 146 pages - only 76 so there's nothing in the manual but I'm guessing the entire manual is available online so I will check that out before purchasing a new pedal.

In your first post you say you have a MOXF8. And you posted in the MOXF forum.
In your second post you say you have a MOX

The MOXF has an 80 page Owner's Manual
The MOX has a 76 page Owner's Manual.

You need to figure out which keyboard and which Manual you have... then note, in either case, I was referring to the REFERENCE MANUAL which is downloadable in PDF format from your Yamaha.com site.

The MOXF Reference Manual is 163 Pages and is what you want read. And while your there download the MOXF Owner's Manual (if you determine that is the keyboard you own) perhaps you didn't know they were different products... the name of the keyboard is printed on the front panel (and the MOXF has those magenta side panels). There is a major difference in the products in terms of specifications, polyphony, Flash expansion, etc., ... (although not where Sustain Pedals are concerned).
For you next pedal, try a Yamaha.

I was just typing MOX as a shortened form to save typing in the whole name - it didn't occur to me that it would create confusion - sorry about that. I have a MOXf8. I also have an S90XS but it's in a different location at the moment so I wasn't able to test it's sustain pedal with my MOXF8. I've used non Yamaha pedals for about 10 years as the Yamaha ones were quite a bit more expensive when I purchased the pedals. I also have a Yamaha WX5F piano so if I want 'proper' sustain I would use that - although obviously not when travelling for rehearsals etc. I will look at Yamaha pedals when I go to purchase them.
Thanks

I was just typing MOX as a shortened form to save typing in the whole name - it didn't occur to me that it would create confusion - sorry about that. I have a MOXf8.

No worries, except the shortened name happens to be another product that actually has a manual with 76 pages. If you do own a MOXF8, do yourself a favor and get the correct manuals... MOXF8 Owners Manual is 80 pages, and the Reference Manual 163.