How hot the hot water must actually get
California rentals must provide hot water, and local building and health codes often set a minimum temperature the hot water should reach — commonly around 110°F, though it varies by area. Water that never gets properly hot can be a habitability problem.
The exact required temperature can depend on local code, but the basic idea is that "hot water" has to actually be hot enough for normal use like bathing and washing dishes. Water that only reaches lukewarm may not meet the standard, especially when the cause is a failing water heater the landlord has not repaired.
If your hot water consistently falls short of a usable temperature, note that when you report it, since it is a real habitability concern and not just a preference. If your water never gets properly hot and your landlord will not fix it, a free review can help you understand your options.
This is general information about California tenant rights, not legal advice. Every situation is different.