How do I fix sony protect error 22?

I have a sony STR-DA3000ES. It is old - barely used. But early on, I received a Protect Error 22 on it. It has been in my closet since then. I have scanned the internet - and nothing has helped me isolate the problem. I have removed all speakers and still get the error code.

I tried to check all output transistors while on the PCB - and they all look the same related to resistance. Hard to really know if they are good or bad as I have not removed them from the PCB - which would be the right way to test them. But they all have the same ohms between all the pins - so my guess is they are OK.

Any ideas on how to narrow down the problem?