Carbon monoxide detected

The Carbon monoxide detected trigger fires the moment a carbon monoxide sensor entityAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more] starts detecting carbon monoxide. Carbon monoxide is colorless and odorless, which makes it one of the most dangerous household hazards because you simply cannot sense it on your own. This trigger gives Home Assistant the ability to warn you immediately, whether your family is sleeping, the kids are playing downstairs, or you are away at work. Pair it with a loud siren and an urgent phone notification for the strongest possible safety net.

Using this trigger from the user interface

If you prefer building automations visually, Home Assistant walks you through this trigger step by step. You pick what to watch, tweak a few options, and save. No YAML knowledge required.

To use this trigger in an automation:

1. Go to Settings > Automations & scenes.
2. Open an existing automation, or select Create automation > Create new automation.
3. In the When section, select Add trigger.
4. Select what you want to monitor. Under By target (see Targets), pick the area your CO sensor is in (like your kitchen or garage). You can also select a floor, a device, a specific entity, or a label.
5. From the triggers shown for that target, select Carbon monoxide detected.
6. Under Trigger when (see Behavior), pick Any, First, or Last to control how the trigger behaves when multiple sensors are targeted.
7. Under For at least, set how long the sensor must stay in the detected state before the trigger fires. Leave it at zero to fire immediately.
8. Select Save.

Options in the UI

Using this trigger in YAML

If you work directly in YAML, or you want to know exactly what Home Assistant does under the hood, this section has the technical reference. It lists the field names you use in YAML, their types, and which ones are required.

In YAML, refer to this trigger as air_quality.co_detected. A basic example looks like this:

trigger: air_quality.co_detected
target:
  entity_id: binary_sensor.hallway_co

This fires every time binary_sensor.hallway_co transitions to the detected state.

Options in YAML

YAML sometimes provides additional options for more complex use cases that are not available through the UI.

Targets

This trigger supports targets. A target tells Home Assistant what the trigger should watch. You can point it at a single entityAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more], a device, an area, a floor, or a label, and Home Assistant watches every matching air_quality entity behind that target.

• Entity: one specific air_quality entity, such as air_quality.living_room.
• Device: every air_quality entity that belongs to a device.
• Area: every air_quality entity in a room or area.
• Floor: every air_quality entity on a floor.
• Label: every air_quality entity that shares a label.

You can also mix target types in one trigger. For example, combine a specific entity with an area to watch both at once.

Behavior with multiple targets

When you target more than one entity (or select an area, floor, or label that contains several), the Trigger when option controls how the trigger responds:

• Any (default): the trigger fires every time any one of the targeted entities transitions. For example, if you monitor three motion sensors in the living room and someone walks past sensor 1, the automation fires. When they walk past sensor 2 a moment later, it fires again. Every individual event counts.
• First: the trigger fires only on the first transition in the targeted group, then waits until all targeted entities have reset before it fires again. For example, if you monitor the same three motion sensors, the automation fires when the first one picks up movement (someone entered the room). The other two firing afterward are ignored, so you get one notification per “someone walked in” event instead of three.
• Last: the trigger fires only after the last targeted entity in the group has fired, meaning all of them are now in the expected state. For example, if you monitor the lights in the living room, bedroom, and hallway, the automation fires only once all three have turned off. This is useful for scenarios like “start the robot vacuum only after every light on the floor is off,” so you know the room is truly empty.

Good to know

• The trigger only fires when a sensor transitions from a known, valid state. If a sensor comes back from being unavailable (unavailable) or having an unknown state (unknown), the trigger does not fire for that recovery.
• Carbon monoxide is odorless and colorless, making automated alerts especially important. Pair this trigger with a loud notification or siren action for maximum safety.
• To react to the opposite transition, use Carbon monoxide cleared.

Try it yourself

Ready to test this? Go to Settings > Automations & scenes, create a new automation, and add this trigger. Save the automation, then change the state of the targeted entity to watch the trigger fire on your actual entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more].

More examples

Real scenarios where this trigger fires in automations and scripts. Copy any example and adapt it to your setup.

Automation: sound the alarm and alert the whole household

Imagine everyone in your home is fast asleep and carbon monoxide starts building up from a faulty furnace. This automation triggers every siren in the house and sends an urgent notification to your phone the instant any sensor picks up carbon monoxide. Those extra seconds of warning protect the people who matter most to you.

• Trigger: Carbon monoxide detected
• Target: All CO sensors (by label)
• Trigger when: Any
• For at least: 00:00:00
• Action: Siren: Turn on
• Action: Send a mobile notification

alias: "CO alarm and notification"
triggers:
  - trigger: air_quality.co_detected
    target:
      label_id: co_sensors
    options:
      behavior: any
      for: "00:00:00"
actions:
  - action: siren.turn_on
    target:
      entity_id: siren.home_alarm
  - action: notify.mobile_app_phone
    data:
      message: "Carbon monoxide detected!"
      title: "CO alert"

Automation: ventilate the garage automatically when CO builds up

A car left idling or a gas-powered tool running in the garage produces carbon monoxide that builds up fast in an enclosed space. This automation turns on the exhaust fan after a confirmed one-minute reading, helping clear the air before the situation becomes dangerous. You could also combine this with a notification so you know to check on what caused the buildup.

• Trigger: Carbon monoxide detected
• Target: Garage CO sensor
• Trigger when: Any
• For at least: 00:01:00
• Action: Fan: Turn on

alias: "Garage ventilation on CO detection"
triggers:
  - trigger: air_quality.co_detected
    target:
      entity_id: binary_sensor.garage_co
    options:
      behavior: any
      for: "00:01:00"
actions:
  - action: fan.turn_on
    target:
      entity_id: fan.garage_exhaust

Still stuck?

The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with the trigger you’re using and what you expected to happen, or share on our subreddit /r/homeassistant.

Related triggers

These triggers work well alongside this one:

• Carbon monoxide cleared - Triggers after one or more carbon monoxide sensors stop detecting carbon monoxide.