@captaintech I'd start off by using Sony's suggestions.
1.1 Power
1.1.1 Will not power on or will switch
off randomly
Check:
Check whether the phone vibrates by pressing the on/off key.
Action:
1. If activation of the vibrator is detected, refer to section 1.4 ‘Display’.
2. If NO activation of the Vibrator is detected, check that the battery is charging when connected to a charger and do a HW master reset.
Follow 1281-3374 Test Instruction - mechanical chapter 2.3.7 (Charging via USB and Easy Charger), Diagnostic Battery / Charging status.
Check:
Inspect the on/off key.
Action:
Refer to section 1.2.3 ‘On/Off Key’.
Check:
Inspect the BtB connector of the Battery to Main PBA.
[image|1030226]
Action:
1. If not properly connected – disconnect and reconnect it.
2. If dirty or oxidized – clean the both sides of the BtB connector.
3. If the BtB connector, the Battery FPC or the Battery is damaged – replace the Battery.
4. Replace Main PBA.
1.2.3 On/Off Key
Check:
Inspect the external area of the On/Off Key.
Action:
If dirty – clean it.
Check:
Inspect the BtB connector of FPC Side Key to Main PBA.
Action:
1. If not properly connected – disconnect and reconnect it.
2. If dirty or oxidized – clean it.
3. If the FPC of FPC Side Key is damaged – replace the FPC Side Key
4. Replace Main PBA.
Check:
Inspect the area and position of the FPC Side Key (Power On/Off) (a) and On/Off Key (b).
[image|1030227]
Action:
1. If dirty – clean them.
2. If the On/Off Key is damaged – replace the Frame Main Assy.
3. If the FPC of FPC Side Key is damaged – replace the FPC Side Key.
You can also check a few other things like this one
[image|1030229]
R1304 is a 0.01 Ohm +/-1% 200.0 mW Resistor