Asus ZenBook 14 UX425EA – Won’t Turn On After Thermal Paste Change

[maxbers]

Hi everyone,

I’m new here and I’m trying to learn some board-level repair skills.

I have an Asus ZenBook 14 UX425EA that won’t turn on after a thermal paste replacement. The laptop was fully working before the disassembly. I only removed the cooling system to clean the CPU and apply new thermal paste.

Current behavior:

• No normal power on
• Battery only: the white power LED turns on for a few seconds, then turns off. No display, no fan activity. I must disconnect the battery or hold the power button for ~15 seconds to get the LED again.
• AC only: the white power LED stays on continuously.
• Battery + AC: the white power LED stays on and the battery appears to be charging.

I noticed that when I connect the battery, there are tiny sparks at the connector. The same happens when I quickly disconnect and reconnect it.

I did not notice any visible damage during the process, but it’s possible I made a mistake during reassembly.

I’ve started basic diagnostics using a multimeter and boardview, and I can take measurements on the motherboard if needed.

Any advice on what to check first, common mistakes after a thermal paste change, or known issues with this model would be greatly appreciated.

Thanks in advance for your help.

[Vusal]

It is normal for sparks to fly when the battery is removed and replaced. After all, there is high current there. Even slight contact causes sparks.

In such cases, the most common problem is that even if a small resistor breaks while changing the thermal paste, it can prevent the motherboard from starting. That broken resistor can carry voltage for an important line. First, check if the 19V main voltage is distributed to the motherboard, i.e., if the input MOSFETs are open. Measure the standby LDO lines. I don't think it's a BIOS issue. But my advice is to examine the motherboard thoroughly with the naked eye or under a microscope. There may be a broken circuit element.