Measure the resistance across the two terminals of the solenoid ,disconnect one of the cables going to it ,after you isolate the electric supply to the shower, before taking the reading. Tell us what the reading is.
If you don't know what setting to use on the multimeter ,show us a pic of it ,and we can advise you.
It doesn’t appear to be. Next try the thermal cut out (tco), it’s a simple continuity check (that’s the one that looks like a wi-fi signal, or a speaker volume.
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