I have a single control moen shower. I guess it's positemp as it turns counterclockwise to adjust temp. It starts from the cold side and goes to the hot side. I only get hot water. When it's on the cold side nothing comes out. I took off cover and there is the center cartridge and a screw on either side (cold/ hot). On the hot side if I turn the screw right or left it turns about a 1/4 turn and slows the water flow. If I turn it the full 1/4 it shuts the flow off. On the cold water side the screw turns and turns and does nothing. Can this be replaced? If so how? Thanks Jim

Those screws are integral stops which are used to shut down the supply of water to the cartridge. Both screws should be fully turned counterclockwise. If you still have problems I suspect debris in the stops and/or cartridge. If you explore further I'd suggest you close your main since the stops may be part of the problem. Just to eliminate any side problem remove the shower head, did that make any change?