If sending a standby command successfully puts the TV into standby but the power-on command has no effect — and the cec-client log shows repeated TRANSMIT_FAILED_ACK errors — this is typically a TV-side CEC implementation limitation rather than a fault with the adapter or libcec.

Why This Happens

CEC power-on commands work differently from standby commands. Standby commands are broadcast to all devices on the bus and TVs reliably respond to them. Power-on commands, however, require the TV to be listening on the CEC bus while in standby — and some TV models, particularly certain Hisense and Samsung variants, do not maintain reliable CEC bus monitoring while powered off. The result is that the TV receives and acts on the standby command but misses or rejects the power-on command.

TRANSMIT_FAILED_ACK in the log means the command was sent but not acknowledged — the TV did not respond to confirm receipt.

Troubleshooting Steps

1. Confirm CEC is enabled on the TV

CEC must be enabled in the TV's own settings menu. TV manufacturers use different names for CEC:

Navigate to your TV's settings and confirm CEC is enabled. Also look for a specific option such as "Power On via CEC" or "Allow external devices to power on this TV" — on some TVs this is a separate toggle from the main CEC enable.

2. Confirm the adapter is on the correct HDMI port

On some TVs, CEC power-on commands are only accepted from the device registered at a specific HDMI port. Confirm the HDMI port the adapter is connected to matches what is configured in libCECTray — open libCECTray, go to the Configuration tab, and check the HDMI port of the TV setting. If Auto-detect is enabled, verify it has detected the correct port number.

3. Try the power-on command from standby rather than fully powered off

Some TVs accept CEC power-on from standby (a low-power state where CEC remains active) but not from a fully powered-off state. Confirm the TV is in standby (usually indicated by a standby LED) rather than having been disconnected from power.

4. Check for a TV firmware update

Some TVs have received firmware updates from their manufacturers that improve CEC compliance. Check for TV firmware updates in the TV's own settings menu.

If Power-On Cannot Be Made to Work

Some TV models simply do not implement CEC power-on reliably — this is a limitation of the TV's CEC implementation and cannot be resolved through adapter settings or libcec configuration. In these cases, consider using an alternative power-on method alongside CEC for other controls:

• IR blaster: An IR blaster can send a power-on signal to the TV using a standard remote code. See Which TV Power Control Method? for alternatives to CEC power control.
• Wake-on-LAN: Some smart TVs support network-based power-on independently of CEC.

Related Articles

• Which TV Power Control Method?
• Getting Started with the USB-CEC Adapter
• Advanced cec-client Commands