A couple weeks ago we realized that there is no cold water in one of our showers. When you turn the shower on, it is fully supplied with hot water. When you turn the handle all the way to the cold side, the water turns off.
Overall cold water supply to the house is fine, including cold water to the other fixtures in the same bathroom, the other bathroom, and the kitchen. So the problem seems to be localized to this specific shower. This morning I tried replacing the cartridge as that seemed the most likely problem, but the same thing is happening with the new cartridge, suggesting that the problem is upstream.
This is an American Standard single-handle faucet. The cartridge type is a 47mm 023529-0070A/H (link).
At the store, I also saw a pressure balancing unit which seems to be a more internal component, and wondered if that could be the problem, but wanted to ask for advice before taking more things apart...
Thanks!