The California Health and Safety Code requires an owner or landlord of a dwelling unit to install an approved carbon monoxide detector device in each existing dwelling unit that has a fossil fuel burning heater or appliance, a fireplace, or an attached garage within the following time period:
By July 1, 2011 for single-family dwelling units.
By January 1, 2013 for all other dwelling units.
A carbon monoxide detector is a device that detects the presence of carbon monoxide gas in order to prevent carbon monoxide poisoning.
Owners and landlords may enter a tenant occupied dwelling unit to install, repair, test, and maintain the carbon monoxide detectors. Of course, the landlord must give advance notice before any such entry in accordance with law.
The full text of the above requirements can be found in California Health and Safety Code Sections 17926, 17926.1, and 17926.2.