Well, fun fun fun.
Downstairs heater is out.
Guests let me know it was 55F this morning. Checked all the usual things, no help. Swapped out an extra control board I had, no help. Gas control valve can't really be tested, just have to swap it. $300-$750 depending on source, Monday. /sigh
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UPDATE:
Take it apart, put it back together, test for current in the right places, bang on it*, curse at it a litte, silently. Turn it all on again. Nothing.
Tap the valve just so...
Viola! Success! Heat!
Time for a Toddy!
*No parts (human or otherwise) were injured in the making of this repair.