L521X Laptop won't turn on

Hi all, 

Bit of a long story here, so I'll try to cut it short.

Have a L521X XPS Laptop. The plastic on the audio output cracked, so we ordered a new audio card. When it arrived we replaced it. Powered up fine. Two days later, it wasn't powering up. We opened it up, disconnected battery, reconnected, and then it powered up again. Two hours later it wouldn't power up.

I took a look at the audio card, and the pins on the one we had been sent were a little damaged. I guessed this was causing the issue, so I ordered a new one. New one got here, inserted it, and the laptop won't power up at all. Here is what happens:

With battery inserted: Orange light indicating charging is lit up. Power button does nothing. Pressing the LED diagnostic on the bottom of the laptop shows 4 white lights.

Without battery inserted: Pressing power button turns on white charging light for 2-3 seconds. Then turns off. LED indicators on the bottom of laptop (diagnostic lights) show nothing when pressed. 

In both cases the fan does not spin up.

So far I have:

• Reseated audio card, ensuring all connections stable
• Disconnected battery and power, held power button for 30sec, reseated memory module, and tried booting it from AC, then battery separately and then together.
• Disconnected modules one by one, including HDD and SSD

no change.

Any ideas?