Hi All, I'm posting this because it may apply to other cyclometers.
First, these are long out of production basic units that for me their small size is very appealing. The problem was that the function buttons became hit or miss whether they worked or not, and when my second one started to act up (no more just swapping one head unit between two bikes) it was repair time.
The repair started with splitting the halves. These have no assembly screw(s) so that meant a press/interferance fit. Gently pry a little at a time all the way around the case until the two halves seperate. Keep track of the very small reset button!
The function keys depress two small dome switches and the top half is held on with cellophane (sp?) tape. remove them and clean the dome's concave side and base with electrical contact cleaner. I also cleaned the contact area for the LCD display. Using new cellophane tape (I used strips cut from packing tape.) reposition the domes to their bases and reassemble.