As indicated above in
red, you need to wait for at least 10 seconds or wait until you hear the relay click before pushing the diagnostic button to prevent this error
.
For the issue with the display board, the use of an electronic contact cleaner as suggested previously in this thread, is ideal.
Fyi, only the top side of the 7-pin header on the mainboard is making contact with the display bd. Make sure the connector on the disp bd is not corroded.
I had a similar issue and a few runs of fine-grit sandpaper on the top side of the header as shown below worked for me.
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