Motor starts for 2 seconds then turns off- why?

I would first check the trigger/switch using a multimeter is resistance mode. The contacts inside the switch can become worn over time and increase the resistance when closed.

If it is a brushed motor then check the brushes and replace if worn.

If it is a brushless motor then it is common for the control boards to blow. These might be available online though when I tried to source one for a makita saw it was almost the same price as a new saw.