I was under the impression, that if there was a leak, the low pres cut-off would have shut everything down before the fusible plug got hot enough to blow.
The lit for the 38SQ says it has Internal current and temp overload (automatic reset), Low-pressure switch located on suction line between reversing valve and accumulator (open 5 +/- 3 psig; close 20 +/- 5 psig), and a High-pressure relief valve located in compressor (450 +/- 50 psi).
Running at 9 psig, that could let it get hot enough.
Valve will not swicth over unless there is refrigerant in the system. If you do ever need to change that valve may god be with you, let us know I'll include you in our prayers!
Yeah, so I hear! At least it is in the indoor compressor box - so they could take the side-panels off or even unhook the refrig lines and move the whole box out to work on it. That should at least make the job less painful.
Just to update, it did turn out to be the reversing valve. The tech was able to get the system running in cooling mode, but it wouldn't switch over to heating. Compressor, boards, etc., were fine - so we opted to replace the reversing valve and be done with it. With all said and done, it only took about two hours plus the initial service call and came out to a very reasonable high three figures.