Well, I finally figured it out after spending the better part of a Saturday. I got frustrated and started unwrapping wires that belong to the Lambda control unit, and behold, the ground wire had been disconnected and taped up under an a/c hose. Sure didn't look like it until I pulled it off. Don't know why I was getting proper signals from the unit when I ran the tests on it. Put the ground back on, and it works now. The relay on the add-on control unit was sticky, so I hit it with some contact cleaner after popping the cover off, and now all is well.