It's taken some time to get back to this, but as it turns out the brass(?) posts that sit behind the hi-limit switch had corroded away, so I replaced them (created new ones from other on-hand spare parts, since last time I tried to order these they sent me the wrong thing). Replaced those posts and the high limit message went away. Of course I blew the inline fuse during testing and got a subsequent low pressure error - a few voltage tests and I knew to check the fuse. So, I replaced the fuse (about $0.60) and the heater starts up again! Thanks for everyone's help.