Yamaha NMAX GPD155 2020 Owner's Manual page 40

Turn the main switch on.
Put the sidestand up.
Push the start switch while applying
either of the brake levers. The engine will
start.
Put the sidestand down.
If the engine stalls:
The sidestand switch is OK.
WARNING
The vehicle must be placed on the
centerstand during this inspection.
If a malfunction is found, have the vehicle
inspected before riding.
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