We have this type of "mechanical" timer switch to control our central heating, which is Potterton combi boiler. The switch keeps failing after 2-3 years. This is our 3rd replacement now. It starts with central heating turning itself on for no apparent reason, although the switch is in off position. The LED indicator also turns on on the switch. After that the switch works intermittently, most of the time you can't even turn it on. Moving the slide switch to ON position won't do anything, not even the LED light turns ON. We don't use the timing programs, only ON and OFF. I checked the pin contacts on the back plate, all seem fine. Next, I opened the unit looking for bad connection and cold solder joints, but nothing popped up. There are a lot of SMD components on the board. I also checked the slide contact for selector switch, no dirt or corrosion was apparent. I cleaned the board with 99% IPA, and put everything back together in hope the it's fixed but to no avail. I noticed the is no conformal coating on the PCB. Could it be moisture that affects the circuit, with the unit being in the kitchen would make sense. I'm gonna coat the board with conformal coating and see what happens.