What does OF.L mean in display?

It means OverFLow - the display unit can read from -39.8°F to 139.8 °F and it is getting a signal from the sending unit that is lower than -39.8 or higher than 139.8. This happens when the sending unit goes haywire (or, not likely, when temperature out doors is REALLY lower or higher than the range). Replace the sending unit. If you really like the product, I suggest you buy another of the same model. It will come with a new sending unit and that will be read by both of the displays if they are within 80 ft of the sending unit. Good luck.