In any home with fuel-burning appliances, there is always the risk that a leak or a device working improperly will cause a buildup of carbon monoxide. This gas can lead to symptoms such as headaches, nausea, and dizziness. In homes where high levels of the gas go unchecked, carbon monoxide poisoning can cause brain damage and even death.

Carbon monoxide is especially dangerous because it is hard to know when it is present. The gas is colorless and odorless, and the symptoms of carbon monoxide poisoning can easily be mistaken for the flu.

For this reason, CO detectors are an essential safeguard. The devices will issue an alarm if dangerous levels of carbon monoxide are detected, allowing homeowners to get to safety and repair the problem. Connecticut law even requires home sellers to guarantee that CO detectors are present in homes where the gas is likely to pose a danger, namely those with fuel-burning appliances, attached garages, or fireplaces; otherwise, they need to give the buyer a $250 credit.

It's easy to install CO detectors and expect that they'll provide protection in perpetuity. But while the devices can work well for several years, their sensors will eventually become less effective or stop functioning altogether.

Detectors that have quietly stopped working give homeowners a false sense of security. They'll believe that the device will let them know if carbon monoxide is present, when in fact it is no longer capable of doing so.

The easiest way to see if your CO detector needs to be replaced is to check for an expiration date. The home safety and security site SafeWise says CO detectors use gels, silica chips, or electrodes to detect changes in carbon monoxide levels. The gas will trigger certain reactions, such as a color change in the gel or a change in electrical currents in the electrode, and sound an alarm.

These sensors can be reset and reused once carbon monoxide is no longer present in the home. However, they will also start to degrade and become less reliable over time. The magazine Family Handyman says gels and metal oxides generally have a longer lifespan, but their accuracy can also be affected by changes in temperature and humidity.

Expiration dates have been a fairly recent addition on CO detectors, so it is a good idea to replace them if you can't find a date on the device. Most detectors will last for five to seven years, although some have advertised lifespans of up to 10 years. Jeanne Sager, writing for the National Association of Realtors, says you can write the date of the detector's purchase on a piece of paper and place it on the device to give you a clear indicator of its age.

Don't expect that the test button will let you know whether a CO detector is expired. Randolph J. Harris, a fire and explosion expert at the Denver forensic engineering company Fay Engineering, says a test will only check to see if the alarm is functional, not the sensor.

Newer detectors take expiration concerns into consideration. Since August 2009, the independent testing certification company Underwriters Laboratories has required all CO detectors to have a warning to indicate the end of its useful life.

When replacing CO detectors, make sure they are placed in areas where they will function adequately. Family Handyman says the gas is about the same weight as air, so detectors will work well as long as they are at least 15 inches below the ceiling. They should also be kept at least 15 feet from gas-burning appliances to avoid false alarms.

Check to see what conditions will trigger the CO detector's alarm. The fire safety and CO detector company Kidde says alarms typically sound after prolonged exposure to low levels of carbon monoxide and shorter exposure to higher levels.

Harris says new alarms often have a threshold of 70 parts per million, which is above the OSHA maximum standard of 50 parts per million. Although a concentration above 50 and below 70 parts per million generally won't be harmful to adults, it can still be detrimental to infants, children, seniors, and people with circulatory or respiratory issues.

If you have more vulnerable occupants in the home or are particularly concerned about carbon monoxide exposure, purchase a low-level detector. Allison Bailes, writing for the home energy company Energy Vanguard, says these detectors can sound if they detect carbon monoxide levels as low as six parts per million.