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From YAMAHA on this issue:
The DECODER OFF message is purely an information message. It can mean one of two things - a two channel source is input or there is no input. In either case the receiver would not need to turn on a decoder to play sound. You can toggle this informational display by pressing the INFO button on the front panel of the receiver or on the remote control (depending on your model receiver) – you will have three choices: DSP Program, Audio Decoder, and Input.
The INFO button basically tells you the status of the receiver. You cannot "turn the decoder on" - the receiver is designed to automatically select and activate (turn on) the proper decoder based on the input signal from the source (DVD player, BD player, cable box, etc.). For example, if your DVD player sends a Dolby Digital signal to the receiver the receiver will see that and automatically activate the Dolby Digital decoder.
If the decoder off message is accompanied by a loss in picture and/or sound, we would suggest double checking all your connecting cables between the source component and receiver. Just one bent or damaged cable or connector can cause a loss in picture and/or sound.
Now the above is straight from Yamaha pointing to what this message means. In short- there's usually nothing ever wrong with the receiver, and it's generally the "Source" causing this.
As a test, select the FM tuner- chances are there will be sound on that Input.
So you would want to go through checking the sources- most commonly the Cable Box has dropped sound and needs to be rebooted. To do this disconnect the cable box from power, then disconnect and reconnect it's HDMI cable on both ends. Make sure the cable's secure, then plug the cable box back in.
Once it's rebooted, check the Yamaha for sound and see what happens.
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