Climate control unit 1991 Wagon.

What does it meant when the degree symbol, and the C or F are flashing on my climate control unit? Got a leaky rad, and there is not much heat coming out of the system for the past couple of weeks, but just the past day or two the little symbol started flashing. I have plenty of coolant in the system, keeping it topped up regularly until the weather warms up enough to install a new rad.

Thanks, Michael.