I am facing the same problem described in this forum for Onkyo TX-NR656 receiver.
After 6 months working fine it starts writting "Thermal Error" on display upon powering up.
Two months later it stars shutting down randomly during a movie play. It could happen the first minutes, could happen after 1h play, 2h play, at a random manner.
Every time it shuts off when trying powering up again, it says "AMP Diag Mode", and then say "CHECK SP WIRE". You turns it off and on again and it continuous playing some minutes or hours up to the next shut off.
I manage to get a service manual in internet and could check that it always shuts off because current protection.
After several tests, using many different volumes, speakers and speakers wires I finally understand that the problem is complete random and happens even if the system has nothing connected to it, not even speakers wires, you just turns it on and waits for a while and the unit will shuts off.
When you turn it on again, after it completes it's Amp Diag test, you are able to checl the protection counters and the current protection counter has been increased by one.
Some times this happen even when you are powering up the unit.
I have updated the system firmware to the latest, dated on 18 December 2018 and did not fix.
I took it to an Onkyo authorized repair service and they told me the problem was the HDMI board. Well I replace the HDMI board and the problem still remains.
Some how the current protection circuit is being triggered, out of nothing being played or plugged to the receiver.
But how can I find out which of the 7 channel is tripping the current protection? Is there any hidden service menu to show that? Is there any one facing this same trouble?
Any idea is welcome.