A1502 Common Issues
Liquid will often corrode the PP3V42_G3H probe point near the hall sensor on the board. It will cause the screen to fade in and out as it is tripping the SMC_LID circuit. Since this point always has power it will corrode away. Instead of removing the sensor board and running a jumper wire underneath, you can run a wire from the trace to the capacitor on the sensor board as shown:
No Keyboard and/or Trackpad not working.
This can include the power button not turning on the MacBook Pro. The Trackpad cable often goes bad due to the bend in the cable. It can have no outward appearance of wear. Replace the cable and it will often solve the keyboard and/or trackpad not working.