The hot water does not come on in the shower (separate shower from tub) with a single moen shower knob. I think maybe the valve that separates the cold from the hot is bad, if this is so how do I change out this part? Our house is only 4 years old.
Thanks,
Tammy Noel

Tammy, doubt that its bad but it may be plugged. You did not mention any other issue so we are going to asssume that all othe faucets work fine, right? Shut off the main supply and then open the faucet. Remove the knob and the bezel. Don't know your model but you should see a cartyridge that can be removed, maybe three screws hold a plate ring? Come back with what you see.