2017 Haibike / Yamaha. I have the same problem currently: intermittent failure of the bike to power up on pressing the power button on the keypad. During static testing I could sometimes replicate the fault by repeatedly pressing the power button. Finally, I could not get the bike to power up, regardless of how many times the button was pressed. Failure is always when switching on. No error messages are displayed. I never experience motor cut-off during riding. After eliminating battery, display and display contacts as the cause (by swapping components), the likely diagnosis was a faulty power switch. The switch function cannot be measured with a meter because the bike works by using an I2C bus. The signalling protocol allows a number of devices (eg keypad, display) to be individually addressed and data sent to them / received from them by the controller. Thus pressing the power button sends a signal to the motor controller (which will be in sleep mode) telling it to wake up and power the rest of the circuitry. What this means is that the fault isn't necessarily caused by a faulty button. Anyway, I replaced the keypad / display /cable harness with a new one. The bike would not power up. I returned the bike to the dealer who refitted the new keypad / display /cable harness. The bike then worked but of course the cause had not been identified definitively. It is likely that the problem will reoccur because, as I said, I experienced the fault when I fitted the new part.
And I checked all connectors and cables very carefully when I did that. Also, it's impossible to connect the harness wrongly. So, it's likely that fault is within the controller. Unfortunately the controller is integral with the motor, so I can't inspect that. I'm told by the dealer that it can take 10 to 12 weeks to get a new motor
. Stevieb's experience may reinforce my view that the motor may have to be replaced. It's a shame the controller and the junction box that feeds it, isn't separate from the motor. I'll update this when I have more information.