ITR told me the fluid should last ten years. I don’t know what post you are referring to, so it’s difficult to answer your question. I don’t believe I have ever had a valve stuck closed, however, I have had one stick open. Hence, the rapping on the valve. I don’t believe I know what you are referring to about the yellow lights for the electrical elements. When I have AC1 and AC2 on the light is either green indicating the elements are activated, or the lights are off because the temperature has been reached. If your onboard water, or incoming park water is very cold, it can be difficult for the electrical elements to keep up with heating the water when showering.
I am assuming the switch you turned on at the distribution panel was the bypass switch? The burner could have activated at this time simply because it was now needed in addition to the electrical elements as you ran your outside shower.

Back in 2021 you commented on a post about the ITR Oasis system. I was 2nd in line tonight and got a cold shower. I had both zones on but where yellow and turned burner on but it did not fire up. I went to my distribution panel and summer loop was green, turned on switch and got hot water and burner fired up as I was testing at outside shower. We are on the road so tomorrow will be a travel day, but if I get into campground early enough I will crawl in there and whack check valve. Do you think when the check valve gets stuck closed it is time for a fluid change. We have a 17 LA and the fluid is starting to get darker. Thanks for any advice. David