Currently, 19 states have passed laws requiring Carbon Monoxide detectors to be in homes and California has been one of those states since July 1, 2011. These laws are long over-due because unlike smoke, CO is odorless, colorless, and will affect you quickly, which makes it a silent killer.
We have noticed recently that many failed inspections are due to the fact there is no CO detector in the home at the inspection time, which results in a re-inspection fee of $150.00, after the detector is installed. People often confuse smoke and CO detectors but they are not the same thing. Smoke detectors have been required for many years but now both are necessary to pass inspection.
The price of a CO detector has dropped since many states began passing laws mandating them and you can now buy them for around $20 at places like Target or Amazon.com.