MX675 multimeter failed

[boumboauto]

Hello,
I got a multimeter from the garbage because it's not working at all. In current and voltage modes it reads 0.00 all the time.
In resistance/continuity mode it can read either 0 and beep when shorting the leads, or OL and beep when leads are disconnected. It's not capable of reading resistances values.
In temperature mode it reads 552C and seems to be maxed out.

Backlight and Display buttons seems functional
Min/max and Hold are hard to tell because it can't measure something so it's hard to hold a value...

I started to open it : it has a board with many op/amps and the hand selector. This board has 12 pins connected to the "motherboard" tasked with display and buttons.

I don't really know where to start as this is very complex, but I have in order of ideas :
- Bulid a schematic as I move and check things up, it will not be complete but help me understand/troubleshoot.
- Check input MOV and protection resistors
- Checking power supplies starting from the battery, to the opamp power supplies, to the microcontroller in order to find if all the 5V rails, 3.3V 1.8V are all "normal"

There is no fuse that I found on the inputs.