The house was built in 2003 and I don't think we
ever tried the shower valve until recently.

It's the single lever type (can't tell brand) that controls both water and
temp. When first opened, cold water runs fine. If you turn the handle all the
way to hot the water stops.

Where to begin?

Pull the handle and escutcheon plate.

Pull the cartridge and inspect for debris. If it's a Moen, check and see if the
long rubber grommets are installed properly. I've seen several where they were
flipped left to right and block the ports.

To pull the cart:
Pull the U clamp horseshoe looking clip, then extract the cartridge. They seldom
just slide out easily. I use a pair of vice grips on the stem, clamped so it's
sticking straight out. First try and pull it out using a little back and forth
wiggle. If not, tap on the vice grips with a decent force, but not too much, if
that makes sense. It will gradually loosen and come out. If you have a slide
hammer you can thread into the stem, that too will work. 10-24 0r 10-32 screw I
think. Moen sells an exytractor that's about $25, but it's junk.