How to Test Your Carbon Monoxide Detectors

Why it matters

Carbon monoxide (CO) is an odorless, colorless, and tasteless gas that can be fatal in high concentrations. Because humans cannot detect it through natural senses, a functioning detector is the only line of defense against leaks from gas furnaces, water heaters, stoves, or attached garages.

Testing your detectors ensures that the internal circuitry, the audible alarm, and the battery are all functional. While many units have a lifespan of 5 to 10 years, according to the National Fire Protection Association (NFPA), electronic failures can happen at any time. A dead battery or a failed sensor renders the device useless, leaving your household vulnerable to a gas that causes permanent neurological damage or death.

How often

You should test your carbon monoxide detectors at least once a month.

In addition to monthly testing, perform a check during these specific times:

• After a power outage: If you have hardwired units, ensure they have switched back to AC power and the backup batteries are still viable.
• When clocks change: Use the "Spring Forward" and "Fall Back" schedule to replace batteries if your units use standard 9V or AA batteries.
• Before the heating season: Test all units in the autumn before you turn on your furnace for the first time.

What you need

• Ladder or step stool: To reach ceiling-mounted or high-wall units.
• Microfiber cloth or vacuum with brush attachment: For clearing dust from the sensor vents.
• Fresh batteries: Usually 9V or AA (check your specific model).
• Manufacturer’s manual: (Optional) To understand specific beep patterns or light codes.
• Canned CO test gas: (Optional) If you want to test the actual sensor, not just the circuitry.

Step-by-step

Testing a CO detector takes less than two minutes per unit. Follow these steps to ensure a thorough check.

1. Notify the household and monitoring service

The alarm on a CO detector is piercing (typically 85 decibels). Warn anyone in the house before you begin. If your detectors are part of a monitored home security system, contact the provider or put the system in "test mode" to prevent an accidental dispatch of emergency services.

2. Clean the unit

Dust and cobwebs can obstruct the sensor vents, leading to false negatives or delayed responses. Use a vacuum brush attachment or a dry microfiber cloth to gently wipe the exterior of the detector. Do not use cleaning sprays or water, as moisture can damage the internal sensor.

3. Perform the "Press-to-Test"

Press and hold the "Test" button on the face of the unit. It may take up to 20 seconds for the unit to respond.

• If it works: You will hear a series of loud, pulsing beeps. This confirms the battery and the alarm circuitry are functional.
• If it doesn't work: Replace the batteries immediately and test again. If it still fails, the unit must be replaced.

4. Check the "End of Life" signal

Check the date on the back of the unit. Most sensors last 7 to 10 years. Many modern units will chirp every 30-60 seconds if they have reached their expiration date. If your unit is older than the manufacturer's recommended lifespan, it will not detect CO accurately even if the test button makes a sound.

5. Using CO Test Gas (Advanced)

The "Test" button on most units only checks the battery and the horn—it does not always check the actual CO-sensing element. To test the sensor itself:

1. Purchase a can of "CO Detector Test Gas."
2. Follow the instructions on the can, usually involving spraying the gas into the sensor vents or using a sealed plastic bag around the unit.
3. If the alarm does not sound within the timeframe specified on the can, the sensor has failed, and the unit must be replaced.

6. Verify Interconnectivity

If your home has interconnected detectors (where one alarm triggers all others), have a partner stand in a different part of the house during the test. Verify that when you trigger the first floor alarm, the second floor and basement alarms also sound.

Safety

• Never use a real flame or car exhaust to test a detector. This is dangerous and can permanently damage the sensor or cause carbon monoxide poisoning.
• Hearing protection: If you are testing multiple units in a short period, consider wearing earplugs, as the high-decibel alarm can cause temporary hearing discomfort.
• Placement matters: Ensure you have at least one detector on every level of your home and outside every sleeping area.

When to call a pro

In most cases, testing and replacing CO detectors is a DIY task. However, you should call a professional (an electrician or HVAC technician) if:

• Hardwired failure: Your hardwired units are not receiving power even after checking the circuit breaker.
• Integration issues: Your detectors are integrated into a complex whole-home automation or fire suppression system that you cannot reset.
• Persistent chirping: If you have replaced the batteries and the unit is not expired, but it continues to chirp or trigger false alarms, have an HVAC pro check your gas-burning appliances for actual low-level leaks.

Cost

• DIY Battery Replacement: $5–$10 for a pack of high-quality lithium or alkaline batteries.
• New CO Detector: $20–$50 for a standard battery-operated or plug-in unit; $50–$120 for smart or interconnected models.
• Test Gas: $15–$25 per can (optional).
• Professional Replacement: $150–$300 (includes the cost of the unit and labor) if you hire an electrician to install new hardwired, interconnected units.