Why does my laptop turn on and then off?Then it turns on and off again

My laptop is actually Lenovo YOGA C740 but I could not find it in the list. I picked the closest to it.

Does it do this infinitely, or does it stop after some time and just shuts down?

Also, how old is it, what OS is it running, has it ever been repaired?

Are all drivers up to date? (many questions i know :))

Hi. this is a Lenovo Yoga C-740-15IML.

History: (long post ahead)

The owner spilled tea on it but not soaked. The owner brought the unit to a service center and was told that the motherboard needs to be replaced.

A few months past the owner finally got the unit back from the service center.

The owner did not use the laptop right away for a couple of days.

When she finally was going to use it the laptop will not turn ON.

She thought the battery just drained so she plugged the charger for a few hours but still the laptop did not turn ON.

She brought it to me. First thing I did was plug-in the charger. The charging indicator lights up for about 7 seconds and then blinks. It goes to that cycle indefinitely.

I opened the unit and I noticed the tea stains inside. But the odd thing is that the tea stain never reached the spot where the motherboard is seated.

I tried draining the units power by removing the battery including the CMOS battery...

Pressed the power button for 1 minute then put everything back except for the M.2 SSD. It turned ON. So initially I thought there's something wrong with the M.2 SSD.

I placed it back inside but the laptop did not turn ON again. So it's not the M.2 SSD.

I tried leaving it to charge for 5hrs. The laptop turned on and it was stable for a few days the owner said. But yesterday while she was using it, the laptop suddenly turned off. Then it doesn't turn back ON again.

Right now I tried disconnecting the keyboard thinking there might be some short circuit that's causing it. But after disconnecting it, still it doesn't turn ON.

I don't know what to do next.

Sounds like there is still a motherboard issue. A decent service centre will warranty their work for at least a month. Did they actually replace the motherboard or simply repair it? Some places will reflow the solder on a GPU, which might fix a problem, but in most cases the problem will come back (a week, a month, a year?) Another thing you might try is to pull the RAM. If there are two sticks, put one back in to one slot and try to start the machine. If it doesn't start, move that stick to the second slot and try again. If that doesn't work, try the second stick in each slot. If one stick works and the other doesn't, you have a bad RAM stick. If one slot works and the other doesn't you have a bad connection between the board and the RAM slot. Also, check the USB ports carefully, any damage/coffee in there? If you have the machine opened up, you might try disconnecting the USB port cable(s) if they are not directly soldered onto the motherboard.