Lenovo ThinkPad X250: Power button LED flashes three times but fails to power on

Lenovo ThinkPad X250 power button LED flashes three times but fails to power on.

To resolve:

1. First reseat the battery and attempt to power on.

2. Remove the battery and attempt to power on.

3. Disconnect the power cable and remove the battery.

4. Turn the laptop upside down and undo and remove the screws holding the base cover.

5. Remove the cover then disconnect and reconnect the yellow CMOS battery as shown.

6. Fit the base cover and power on.