When lukewarm water counts as a violation
Water that only gets lukewarm can count as a hot-water problem, because a rental has to provide water that actually gets hot enough for normal use. Lukewarm-only water often signals a failing water heater the landlord must fix.
"Hot water" means water that reaches a usable temperature for bathing and cleaning, not just slightly warm. If your water never gets properly hot, the cause is usually a broken or malfunctioning water heater, a bad thermostat, or sediment buildup — all the landlord's to repair.
Note the temperature problem clearly when you report it, and describe how it affects daily use, since "lukewarm" can be easy for a landlord to brush off. A consistent inability to get hot water is a real habitability concern, not a minor complaint. If your water will not get properly hot and your landlord will not fix it, a free review can help.
This is general information about California tenant rights, not legal advice. Every situation is different.