$400 to change a shower cartridge!!?? Find someone else.

Best to be able to give them the make and model ahead of time so they have the cartridge on hand when they arrive.

So the price is wrong; is the diagnosis right? Would a bad shower cartridge cause the temperature problems?

Thanks.

What brand is it? It may be the problem it's hard to tell, you may just need a line flush by just removing the Cartridge. Changing the Cartridge should take less than a half hour plus cost of Cartridge, usually under $40 for a Cartridge. $400 is very high for changing a Cartridge. Some brands will send you a replacement for free if you are the original purchaser of the Shower Valve and Cartridge.

keep in mind that many plumbing manufacturers have a lifetime parts replacement policy

look up your model and call them!