Overview

The troubleshooting steps here are ordered by most-common to least-common cause of issue.

Display Settings

Dig through the settings in the Display to enable all CEC functions.
Contact the manufacturer to confirm that the display supports the desired function; most displays we have tested will respond to Power On, Power Off, and HDMI Selection. Past that, it is very hit-or-miss.

CEC Codes

Be sure you are using the correct CEC codes. Built-in and custom codes can be found at the CEC over IP Control article.

Lazy Responses

Some endpoints seem to be 'lazy', meaning they do not respond to the first command sent after a long idle time. If dealing with a 'lazy' endpoint, send the command back-to-back with a short delay between.
Example:

1. Send the HDMI Input command - \xEF\x82\x10\xF0
2. Wait 200 ms
3. Send the Power Off command - \xE0\x36