Thanks Patrick for this first help.
If it's really the engine button, check the 4 solder points circled in orange of the attached picture (next time i'll use a thicker brush).
Also pin 9 of the resistor network should be at 3.3V if the button is not pressed and set to GND when button is pressed.
Then you can check the soldering of the GND point near the orange: button and the HC165 CI.
And last (hope not), check the resistor network are in the right direction.
Let me know,
Xavier