What are the symptoms of a bad thermostat?

If you are asking about a Honeywell T87 home thermostat with a Q539A sub-base this might help. With the stat. in cooling mode. the temp. in house will get to warm before A/C kicks on & it will get to cool in house before it turns off. If this is the problem the sub-base is bad. There is many more problems that can be caused by this stat. But this one is overlooked by many repairman. Changing Q539A will fix problem.