I recently had some technicians check out my Climatemaster tranquility series vertical stack water-source heat pump (composed of chassis and cabinet), because it was cooling my condo down only to about 75-77 degrees F. Based on two numbers being close together (possibly voltages or refrigerant pressures) and a history of bad thermistors on these models within my building (mine was replaced two years ago), the techs assessed that the problem could potentially be solved by replacing the thermistor ($350) or, more likely, by replacing the TXV ($950).

I'm considering the following options and would appreciate recommendations and any other insight:

1. Replace either thermistor or TXV and hope that does the trick
2. Replace both to make sure the problem's fixed
3. Buy a new chassis (is this possible? would a new chassis include new thermistor and TXV parts?)
4. Buy an entirely new unit (might have to knock out some drywall if the cabinet cannot be removed via the cutout already there)

I don't know what size the unit is, my guess is 3/4 ton (2.6 kW) since that's the smallest available (my condo is small) for that particular model. Part of me wants to buy an entirely new unit from another maker since I don't really trust Climatemaster to be reliable, but I don't know if I have a choice. Also, for reference, the thermostat is Climatemaster.