Sounds like thermostat is stuck open
Yep, it was. Two bolts hold the flange that the thermostat is under . . . seals with an o-ring. Sounds like a 10 minute job doesn't it? It ain't.

But, the parts were cheap enough and I didn't break anything else while I was fixing it, so I'm happy. It warms up and sits on 190 now and the the heater works again!