Septic maintenance is the landlord’s duty
The landlord is generally responsible for maintaining and repairing a septic system serving a rental, and a failing septic system that causes backups or surfacing sewage can be a habitability problem.
In homes on septic rather than city sewer, the tank and drain field are part of the property the landlord must keep working. Warning signs of trouble include slow drains, gurgling pipes, sewage odors, backups indoors, or wet, smelly patches over the drain field.
A tenant is expected to use the system reasonably — not flushing things that can harm it — but maintaining and repairing it, including pumping and fixing failures, is the landlord's job. A neglected septic system can create real health risks and make a home hard to live in. If a septic problem is affecting your home and your landlord will not address it, a free review can help.
This is general information about California tenant rights, not legal advice. Every situation is different.