Product review (5 stars): Fixed my problem!
This replacement solenoid for my Honeywell TrueSteam humidifier worked like a charm. I recently had a problem with water overflowing the humidifier. The troubleshooting section of the manual gave a procedure for dealing with the six blinking red lights on the front panel, which included replacing the water sensor. I did that, but it did not fix the problem, so I went online again and did some more research. Others had reported that they had to replace the solenoid valve. (My guess is that some scale got caught in one of the valves, because I have not been good about changing the water filter regularly.) I called my local HVAC dealer and they estimated $300-400 to fix it, but the part itself is only about $60 so I decided to try and replace it myself. It was a little tricky because I couldn't find any instructions, but it's not overly complex. You just have to keep track of what plugs into what as you take it apart.

In any case, replacing the solenoid took me nearly an hour without instructions--I was being careful not to break anything, and I did take some time to clean out the water tank again. When I put it back together, it took a little while for the system to recognize that the humidifier was back online, and then it had to fill the tank and start heating the water. After an hour or so I checked and found that everything was working normally again. (Whew!)