How to Test Carbon Monoxide Detectors

Why carbon monoxide detection matters

Carbon monoxide is a colorless, odorless gas produced by anything that burns fuel: your furnace, water heater, gas stove, fireplace, car engine, and generator. In a properly ventilated home, CO dissipates safely. When ventilation fails or an appliance malfunctions, CO concentrates indoors.

At low concentrations, CO causes headaches, dizziness, and nausea. At higher concentrations, it causes confusion, loss of consciousness, and death. The CDC reports over 400 CO deaths per year in the U.S. and more than 100,000 emergency visits.

A working CO detector is the only way to know CO is present before symptoms start. Monthly testing confirms your detectors are ready to alert you.

What the test button actually tests

Pressing the test button checks three things:

1. The battery or power supply. The detector needs enough power to trigger the alarm horn.
2. The alarm horn. The piezoelectric horn must be loud enough to wake someone from sleep (at least 85 decibels at 10 feet).
3. The sensor circuit. The button triggers the sensor circuit to confirm the electronics can process a signal and activate the alarm.

One thing the test button does NOT check is whether the electrochemical sensor itself still responds to actual carbon monoxide gas. The button bypasses the sensor and directly triggers the alarm circuit. This is why the sensor's age matters. After 5 to 7 years, the sensor can degrade to the point where it would not detect real CO even though the test button sounds fine.

There is no consumer-grade way to verify the actual sensor is still responsive without exposing it to CO gas, which is dangerous. Instead, rely on the manufacturer's stated service life and replace units when they expire.

CO detectors vs. smoke detectors

Some combination units detect both smoke and CO. These are convenient but have different lifespans for each sensor. The smoke detection component lasts about 10 years. The CO sensor lasts 5 to 7 years. If you have combination units, replace the whole unit when the CO sensor expires (the shorter lifespan) unless the manufacturer states otherwise.

Keep them separate if you prefer to replace components independently. Standalone CO detectors cost $20 to $40 each. Combination units cost $30 to $60.

Where CO risks are highest

Some areas of the home have higher CO risk than others. Make sure these have a detector nearby:

• Attached garage. Car exhaust is a major source of residential CO. Even with the garage door open, CO can seep into the house through shared walls and doors.
• Furnace and water heater area. Both are combustion appliances with heat exchangers that can crack and leak CO. The detector here serves as an early warning before CO spreads to the rest of the house.
• Fireplace. A blocked chimney or poor draft pushes CO into the living space instead of up and out.
• Near bedrooms. CO is most dangerous when people are asleep and cannot recognize symptoms. Install detectors within 15 feet of bedroom doors so the alarm wakes occupants.

Seasonal testing is extra important

CO risk increases in cold months because furnaces, fireplaces, and gas appliances run more frequently. Test detectors at the start of heating season (October or November) and do an extra check mid-winter. If you use a portable generator or a fuel-burning space heater during winter storms, verify that CO detectors near those devices are working before you start them.

How this fits into a maintenance routine

CO detector testing is a monthly task that takes under a minute per detector. Pair it with smoke detector testing since the process is identical: walk the house, press the test button, listen for the alarm. If you test both on the same day each month, the whole house check takes 5 minutes at most.