Things to check
Check for water
If water gets into the plug points it can cause the keypad to register different keys than expected. There are 2 points to check; the connector between the keypad and the cable (requires a small phillips head screwdriver to open) and the plug used to move the keypad to another location. Undo each of these points and check for significant moisture. If either of these points are wet, dry them out and try again.
There will be some clear silicone grease in these points. Do not clean it out.
Check other gear
If you have a second keypad, try plugging this into the same point and see if the problem persists.
If you have a second keypad position, try plugging the problematic keypad into this position and see if the problem persists.
Check if the pullcord is working correctly.
If you are still having problems