A shower that won't turn off usually means a bad cartridge. But before you start pulling trim plates and trying to find the manufacturer, try this:
Turn the water off at the stops (if it has them). If it doesn't have stops, kill the whole house. Pull the handle and trim, then put a towel under the valve body. Take a flathead and gently pry the clip out. Here's the trick: spray some CLR or white vinegar around the cartridge body where it meets the valve. Wait 5 minutes. That mineral buildup is what makes cartridges seize up.
9 times out of 10, that soak turns a 30-minute fight into a 5-minute swap.
Question: What's your go-to trick when a cartridge is seized? Drop it below.