The Hygrometer on my Air-O-Swiss U7142 reads 25% all the time. I assume its not functioning properly as I have other hygrometers in the house which say higher readings. Is there a known way to fix this? I have, just now, unscrewed the machine apart and cleaned it with air and a cloth.

I have got the same problem with my u7142 unit. This most probably means that the hygrostat inside is either broken or it has been heavily covered with the "white dust" in case the demineralization cartridge has not been changed for a long while.
To resolve it you have two options: either clean the humidity sensor or replace it. The sensor is at the bottom of the front panel, so you need to disassemble the front panel only, no need to open the whole unit (note that there are 2 screws at the bottom holding the front panel also). You need to get the sensor out from the printed circuit to open the cover and clean the surface of the sensor with something (blowing air does not help; you need physically touch the surface with something). If it does not help then you can go for the second option to replace the sensor, the type used in the unit is SYH-1. It is not a pricy element so you may go straight to the replacement if you find a shop nearby to buy it. I have simply replaced the sensor and now the unit is showing the proper humidity value and adjusting the steam automatically again.

I had a similar problem with the humidity sensor on my Air-O-Swiss 7135 humidifier. Here is what I did to fix it:

1. Unplug the unit, remove the water tank, and drain all the water
2. Unscrew the five torx t20 tamper proof screws from the bottom of the humidifier and remove the plastic cover. This has to be done with the humidifier in the upside down position or components might fall out
3. Unscrew the philips screw holding the small chip board with the hygrometer on it. This small board with the hygrometer on it is located right next to the vent holes and the exit hole for the power cable
4. Now that the chip board comes loose, unclip the wires connected to this chip board, this will completely disconnect this chip board from the humidifier
5. The hygrometer is enclosed in the plastic upside down U shaped object protruding from the board. This U shaped plastic cover is about 1.5cm tall by 1cm wide. You have to remove this cover to reveal the hygrometer. The cover is held in place by three clips, un clip one of the corner clips (left or right) and the whole cover will come off.
6. The hygrometer is the small chip that is underneath the plastic cover. There is a resistor circuit printed on this chip which you should be able to see as copper lines. Gently clean this using a cloth or tissue and distilled water. Use distilled water because it has no impurities and will dissolve the salt deposits that might have accumulated on the circuit.
7. Let everything dry and then re-assemble the humidifier.
8. Switch it on and the hygrometer should be good now.

I replaced the sensor with an Amphenol HS12SP, and it works well.

The screws holding the front panel are a Torx head with a nasty pin in the middle to keep us civilians out. I drilled out the pins, but wasn't able to avoid taking a little off the Torx teeth. Fortunately I had a flat blade screwdriver that turned the screws.

The leads on the HS12SP are spaced a bit further apart than the holes on the circuit board. It can still be installed securely, just sticks up a little bit. When I first turned the unit on with the new sensor, it read in the 60's in a room with 44% humidity, but after an hour or so of operation it settled down.

Edit: I had been critical about the reading's accuracy, but after using it for a while I see it does closely track a humidity meter placed next to the sensor. When initially turned on, the unit reads high, but after a minute or two it settles in. Pointing the nozzle away from the front panel helps reduce the variation.