So if power is ok in other printer something is overloading it in the first printer.
If error code is not coming up without the print head then the case really seems to be so that the processor board is the one that is overloading.
It's also possible that some kind of an "all good" signal is missing.
But it also seems that the head itself has its power components, so with luck the repair is easier than very difficult, very likely next head is also needed.