Er or -- (2 dashes)
Means bad wire
There are four numbers in the display of the CoolBot
The first two numbers are the temperature in the room
The second two numbers are the temperature on the FINS.
If your frost sensor wire is a heavy gray wire with a black shrink-wrapped tip it's often just a drop of moisture in there, and using a HAIR DRYER to heat up the TIP end of the sensor will evaporate the moisture, then use a super-thin smear of silicone to seal it back up. You want to get the tip quite warm for about 90 seconds but not so hot that it's going melt anything. Then let it cool and see if it starts displaying the correct temperature. (Compare the room and frost temps in the display, they should be within about 3 degrees of one another)
If your wire is a thin black wire with a hard plastic tip and it's reading ER or -- you will need to replace the wire, that one can't be repaired.
You can order a replacement sensor which can be found here: https://storeitcold.com/shop/product/old-style-sensor-wire-comes-off-the-top-of-coolbot/