I've noted folks struggling with this issue, here's a fix I posted 11 years ago. Happy to say my 4 button thermostat is still working flawlessly at 26 years of age. Sadly, one of the original Penguin ac units has given up the ghost. A bit of additional info:
About five years after I performed the fix linked below the thermostat started requiring fiddly button presses. Rather than rebuild the buttons again I resorted to spraying some contact cleaner on them, {after removing outer cover so physical buttons were exposed) and it straightened up and flew right. Please note the contact cleaner used was Deoxit D5, specifically NOT CRC or similar. Theres a reason Deoxit is ten times the cost of garden variety contact cleaners and is specified for many aerospace, high end electromechanical applications, irreplaceable high end legacy audio preamps etc. It stands alone in effectiveness and leaves no goo behind. No interest in company personally, but would not, personally, leave the dock without it.
How to repair your Dometic 4 button thermostat (https://www.foreforums.com/index.php?topic=15826.0)