Agree with @coleman_ that replacing the start button with a new switch is better than swapping. Actually any new press button would do. I fixed by replacing the start button and adding 104 capacitor across the new button to reduce some noise. After reading the PCB traces, it seems that Breville uses voltage devide circuit and one ADC pin to catch signals for all buttons:
[code]
VCC
|
1kohm
|-- ADC pin on MCU
|
1kohm -- start button -- GND
|
1kohm -- convection button -- GND
|
1kohm -- other button -- GND
[/code]
When different buttons are pressed, ADC will sample the voltage and get different readings, but then the start button is aged and the noise or higher resistor has caused the voltage to be mis-classified as the convection button.
That is why the workaround worked "Press and hold the convection button then press the on/off button, and lastly release the convection button".
The reason the "butter knife" workaround worked sometimes, was because the control pannel was not grounded so it has some induced voltage (which would trigger voltage tester pen).