ecobee3 lite will not connect through Matter, HomeKit or another platform

The checks in more detail

It works with Apple HomeKit and other major platforms, and which platform features it offers depends on its generation and software, so check ecobee's compatibility information for your exact model before assuming Matter support. Start with power, because a thermostat that is not stable cannot stay in a smart-home system. The ecobee needs steady power from a common (C) wire or from the included Power Extender Kit if there is no C-wire; a thermostat that reboots, loses Wi-Fi or will not power on is very often a missing or incorrect C-wire, and a pairing will not survive it.

Confirm the thermostat is online in the ecobee app, with a good Wi-Fi signal, before you add it to HomeKit. To add it to HomeKit, open the Apple Home app, tap Add Accessory, and scan the HomeKit setup code shown on the ecobee's screen in its settings, or printed on the device. Keep the iPhone on the same Wi-Fi network as the thermostat, with Bluetooth on, and make sure that a Home hub, an Apple TV or a HomePod, is set up if you want remote control and automations.

If the thermostat does not pair, restart it and the router, check that the thermostat is not already paired to another Home, and remove it from the old Home first. If it was working and stopped, check the Wi-Fi, the C-wire power, the Home hub and the ecobee software version, then remove the accessory from the Home app and add it again. A router that isolates devices or blocks local discovery can stop HomeKit discovery. For a Matter connection, check that your model and software offer it. Contact ecobee with the serial number if pairing fails.