I have had a ClimateMaster Geo-Thermal System - Model TTV064AGC01JLKS with a ClimateMaster 433J Programmable Thermostat for 5 years and it has never worked correctly - current situation is as follows:

The system does not respond to any of the thermostat commands e.g. continues to cool after "set temperature" has been
reached. Changing the mode to "off" manually at this point causes system to switch over to heating.

Settings are currently: "COOL", "AUTO FAN" --- Set Point Temp at 78, room temp continuing down past 72.

Anything a home owner can do to either diagnose of fix this problem?

p.s. Thermostat and back plate have been replaced three times along with some solenoids and a valve has been cleaned.

HVAC installer stumped.

Thank you