I went to use my trusty CTEK last weekend and found it wasn't in the proper mode and the button wasn't responding. A little research turned up that this is a fairly common problem as the units age. The fix is fairly simple fix - posted below with limited editing (stealing shamelessly). Worked for me.

The Mode button failure often starts out by the CTEK charger responding slowly to inputs, then very erratically, then it stops working completely. At that stage the charger is useless. The reason is because inside the unit, below the Mode button, is a very small micro switch that fails with the smallest amount of dust or dirt. The rest of the unit is fine. The simple fix:

**** Make sure that the unit is not connected to the mains supply throughout this procedure. ****
1. Unscrew the 4 back plate screws and remove the back plate.
2. Loosen the circuit board, flip it over and expose the small circular micro switch plate that sits below the Mode button. It takes a bit of fiddling to loosen the board, including releasing one of the external cable connections. This does not mean disconnecting it from the circuit board, but where the cables enter the unit at either end. One of those collars is designed to pop out by rotating it and pulling gently at the same time.
3. Spray the micro switch thoroughly with contact cleaner (for example Servisol). It's mostly alcohol and will evaporate quickly.
4. VERY IMPORTANT..... Press on the circular micro switch plate and rotate it to-and-fro several times through 180 degrees.
5. Reposition the circuit board and replace the back plate.

Here's a video from a different source:


If that doesn't work, there is a more involved process to remove the switch, clean, and reattach it with superglue, but I didn't have to go that far.