My Classic Thermapen started to not come on sometimes. I sent it in, had it evaluated and repaired. The tech I dealt with said he could rebuild it to better than original condition. I went with the repair. That
was a few years back. Still works well to this day.
Suggestion - If your Classic is beat up to the point of costing more than a minor repair, go with the MK4. It is just so much better: 1 AAA battery that has 2X the life and ~ 1/10 the cost over the 2 button cells.
Waterproof vs resistant. Display light. Auto rotating display. Better auto off features - simply touch the unit to turn it back on vs close + reopen (saves wear + tear). The downside, it costs just a bit more.
More than offset by its features. I have both Thermapens. Had them for some time now. Forgot. If you go with the MK4 and get the protective cover, it has a tether/string that allows you to hang it up.