It's unlikely to be a throttle issue--if it were, the throttle probably wouldn't work at all.
It's more likely to be a setting in the controller (regardless of what you set things to with the display) that is requiring detection of the PAS sensor rotating (in the correct direction), probably also requiring detection of the motor spinning (in the correct direction), to allow the throttle to work. WHen it doesn't get all the right conditions, it shuts down.
Walk mode works because that doesn't require any PAS input since you wouldn't have feet on pedals while walking the bike.
I'ts possible the controller setting for the PAS requirement can't be changed, even though it may show that it's changed in the LCD. There've been troubleshooting threads with various controllers that seemed to have this issue. (not really a problem while riding, but can be durign setup/testing).
But the PAS can be spoofed with a steady pulse train, like from a 555 timer (google will show you dozens of schematics/etc for those), that you can use a pushbutton to emulate so you don't have to use the PAS sensor, at least while testing. It just has to be a square wave at 0-5v at the frequency the PAS sensor puts out while cranks are turning. Probably a few Hz at most, maybe only 1.
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