I've got a Delta 1700 series shower, and it's been running out of hot water more quickly over the past few months. Had a plumber come by, and he recommended replacement of the water heater (in retrospect, without specifically checking the issue with the shower, given that other faucets worked fine), but installing a new 40 gallon heater hasn't solved the problem. Had the plumber come back, and he said that the problem was with the valve cartridge, which he quoted for a $500 replacement. After some searching on here and elsewhere, I figured this was the right diagnosis and that it was an easy enough replacement to do on my own.

However, when I called Delta to arrange getting the replacement part under warranty, the Delta representative was convinced that it couldn't be the cartridge, since (in his words) a faulty cartridge would result in no water coming out, and not just a lack of hot water. He didn't offer any further advice other than to remove the cartridge and see if it rattles, which would indicate a working pressure balance spool. What do you think? Is he right? Everything I've seen elsewhere points to it being the cartridge. And if it's not the cartridge or the heater, what could it be?