90 Le Sabre heater problem-help!!

My Mom's 90 Le Sabre has intermittent heater a/c operation. The heater fan works but no warm air. If she leaves the thermostat on 90 degrees, it will usually work, too well, but at the beginning of the season or, now after the battery was changed, no heat at all. Without having to change a costly circuit board or electronic assembly, her mechanic will go under the dash and do something to get it to work [maybe manually do something to get heater core to open and circulate?] He is not available till January.

Does anyone know what he does to bandaid her problem? She will be taking a trip on Friday before Christmas and I'd like to get her going. Thanks

There is a door under the dash which is supposed to automatically control the source of the air entering your car. With the door in one position, air flows over the heater core before entering your car vents. With the door in the other position, air passes over the air conditioner evaporator before coming into your car vents. The door may also be in some intermediate position to allow a mixure of warm and cooler air.

It sounds like he may be planning to manually force the door so the heat from the heater core always flows through the vents. While this may be OK for the winter, you may not want this in the summer. I would ask.