What is the "Call For Service / Service / Malfunction / MALF" sign on the screen?

"Call for Service" or "Malfunction" displayed on a thermostat screen indicates that the heat pump system is sending a signal (voltage) to the thermostat terminal marked "L".
However, it does not suggest a malfunction in the thermostat itself. In most cases, the indication is caused by a compressor protection device that closes a switch that directs power to "L". In a few instances, power to "L" has been caused by voltage trickling through the HVAC circuit board while the system is running normally.
Furthermore, the manufacturer of the heat pump system can determine the likely cause of a Call for Service or MALF indication on the thermostat screen by identifying the component in the system that sends  power to the "L" terminal.