Try this.
Disconnect all power sources.
1. Hold the 'option' key down.
2. Plug in the power source.
3. Keep holding down the 'option' key for about 5-seconds and then release.
4. Disconnect all power sources and wait for 10-seconds.
5. Plug in power source and power on computer.
This has worked for me on countless occasions.
Thanks to the guys at Stockley Park that told me this years ago.