Just a couple suggestions to anyone doing this mod (btw it is a great one). If you are getting the "Hi F", chances are that you may have a cold solder. That means that it looks like it is soldered but it is not actually making contact. Try undoing the solder joint, and redoing it. Also, the bad temp reading and possibly the same "Hi F" error are due to polarity. Most electrical temp probes are made of two types of wire (they will look the same but are different materials). Without going into why they have two types of wire lets just say the legs of the sensor need to go back in the same hole of the circuit board. I would need to open one up when I get home to see if that is this case. Good chance it is.
If you are worried about wether or not your hygrometer is accurate here is a link to a simple test you can perform at home. It is very similar to how they are calibrated and set up in an industrial setting. BTW, I calibrate laboratory
instruments for a living.
Hygromter Calibration
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