What to do if after cleaning the motherboard the m31s does not turn on

HI @mariuszogorek

Didn't the service centre fix and test the phone to see if it worked after it was dried and cleaned?

Here's the SM M317F Troubleshooting guide that may help to find out what's wrong.

Follow the flowcharts from p.9 → and see how far you get through them.

You can use the search function in your computer's browser e.g. in Windows PC press Ctrl +f keys (both together) to locate the components mentioned in the flowchart in the board layout diagrams in the guide. You will need to zoom in a lot - 500%+ to see them.

Hopefully a start in finding out what's wrong.