I have been using TCL + Marantz with ARC set. When I play a video on the TV, I choose HMDI 1 (ARC Input), and the sound is routed to the AVR.
Though you may have done all or some of the following, here is what I did.
(1) This is important. You have to use ARC capable HDMI cable. Ordinary HDMI cable will not work. Locate the ARC HDMI input on both devices and connect them using the ARC enabled HDMI cable. In most cases it will be HDMI 1.
(2) In TCL TV, go to Settings >> More Settings >> Device Preferences >> Input. There you will find the 'Consumer Electronic Control' Sub heading. In that you will find three options: (1) HDMI Control (2) Device auto power off (3) TV Auto Power On.
Set HDMI Control to ON position.
The other two are optional. If (2) is set on, the TV will switch off the AVR when you power it down. Similarly, if (3) is on, the AVR will switch the TV on when it is powered on.
(3) On the AVR, you have to set 'HDMI Control' and 'Use ARC function' to On position. Usually if you switch HDMI Control to On, the ARC function automatically turns on.
(4) With all these all set, power down the TV and AVR and power them on again.
(5) The simplest way to check whether ARC is working is to adjust the volume on the TV. If CEC and ARC are working, the TV will display the name of the AVR, and volume position of the AVR. Not it's own. If this is not shown, then the CEC and ARC are NOT working.
(6) You can also adjust the audio to the AVR through ARC to PCM, Pass through and other settings.
Correcting Errors
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If after you do all this and the system does not work, here is what you can do.
1. If you have not used a ARC enabled HDMI cable, that is the first step to correct.
2. If are using a ARC enabled HDMI cable, simply reverse the cable and try. That may work.
3. Try a different and 'faster' ARC enabled HDMI cable.
4. Call the TV or AVR technician.