Who fixes a broken smoke alarm
It depends on why it is not working, but a broken smoke alarm is a serious safety issue either way. In California, landlords must provide working smoke alarms, and they must repair or replace one that fails after you report it.
If the alarm is dead because of a bad unit, faulty wiring, or age, that is generally the landlord's responsibility to fix. If it simply needs a new battery, tenants are usually expected to handle that small step — though many landlords will still replace batteries on request.
Either way, do not disable or remove an alarm to stop it from chirping, because that leaves you without warning in a fire. Instead, report the problem to your landlord in writing and keep a copy.
A working smoke alarm is often the difference between a close call and a tragedy, so this should not be left unresolved. If your landlord ignores a report that an alarm is broken, a free, confidential review can explain your options.
This is general information about California tenant rights, not legal advice. Every situation is different.