The answer to 'why is my carbon monoxide detector dimming lights' is that a carbon monoxide (CO) detector can cause lights to dim when it is triggered. This is because CO detectors draw a significant amount of power, which can temporarily overload the electrical circuit and cause a voltage drop that makes the lights appear dimmer.
When a CO detector is activated, it triggers an alarm and also powers up internal components to measure and monitor CO levels. This sudden spike in power draw can temporarily reduce the voltage available to other devices on the same circuit, including lights, causing them to dim or flicker. This is a normal safety feature to alert you to a potential CO leak and should not be ignored.