It stops by passing of cold into hot and visa vera
You need to check if the dip tube in the hot water tank is busted, that the usual cause
If you mean shower valve, screws are for shower valve coverplate.
You have to replace the cartridge
You can get a back water valve. It is similar to a check vale/ the back water valve goes on your main sewer line out side of home. It is desined to stop black water form coming into your home if main line stops up. Thay do make a device that can be installed into an exsisting drain it lets water through your drain but will not let water back up through you shower drain.
Behind shower valve handle and trim plate
Back water valve
Cut out the drywall in the wall behind the shower. Replace the valve. Patch the drywall.
Check the angle stop. Check the supply line. Then check the fluid master valve.
The rubber washer in the hot/cold valve needs to be changedANS 2 Dismantle the diverter and change the seals.
A leaking shower valve by installing a new cartridge.
Check your EGR valve Check EGR valve
Oil check valves are designed to stop the drain-back of oil. A replacement valve costs between $25 and $30 depending on where you buy it.