Simple clog vs. a structural problem
It depends on the cause. A simple clog a tenant caused — like hair or grease in a single sink — is often the tenant's to clear, but clogs from aging pipes, tree roots, or the main line are the landlord's responsibility.
Minor clogs in a single fixture are common, and a tenant can sometimes handle them with a plunger or drain tool. But when a drain clogs repeatedly, several drains back up together, or the blockage is deeper in the plumbing, the problem is usually structural — and that is the landlord's to fix.
If a plunger does not solve it and it keeps returning, treat it as a plumbing problem rather than a routine clog, and report it in writing. If your landlord blames you for a recurring clog that is really in the building's pipes, a free review can help you sort out responsibility.
This is general information about California tenant rights, not legal advice. Every situation is different.