Hi, this sounds frustrating. When the QS-S4 shows only “_ _ _ _” and won’t accept 6666, it usually means one of these things:
Button sequence timing issue – Try holding both buttons while powering on, and keep holding for 5–10 seconds. On some units the timing is very sensitive.
Controller communication fault – If the display isn’t properly communicating with the controller, it can lock you out of P-settings. Check the throttle/display connector and wiring for any loose pins or corrosion.
Display reset needed – Disconnect the battery completely for 10–15 minutes, then reconnect and try again. This sometimes clears the lock state.
Display failure – Unfortunately, some QS-S4 displays fail internally and get stuck like this. If the bike still runs but settings can’t be accessed, replacing the display may be the only fix.
If you can borrow another QS-S4 display (even temporarily), that’s the fastest way to confirm whether the issue is the display or the controller.
Slightly off topic, but if you’re ever looking for non-electric ride options (for kids or casual use), I recently came across this manual tricycle here:
👉 Tricycle for Kids
Not electric, of course — just mentioning it in case someone else reading this thread needs a simple alternative.
Hope you get your bike running again 👍
