Do the easy stuff first! Is this a new install or has it been working well for a time and now it is not? Is the entire unit continuing to run or just the fan? Thermostats generally turn the unit on and off when the desired temp is reached but the fan itself (not the compressor or heat strips) typically operates in 3 modes, off, on or auto. check the fan setting.
It's not a new install. The entire unit is continuing to run, it will not shut off when it reaches the requested temp. It also won't turn off at all.
Is it the thermostat or the relay switch that needs to be replaced?
The thermostat sends a signal to the relay to tell it to actuate and supply power to the compressor. If you have and know how to use a VOM, you can see if voltage is present at the relay. If there is voltage at the relay (the signal part) then the thermostat is improperly supplying voltage. But if you are saying setting the thermostat to OFF is not suhtting down the unit, then most likely the relay has failed in the closed position. Relays are usually cheaper than the a thermostat so you may be in luck! Make sure you properly secure power to the unit if you attempt to replace the relay yourself. I recenlty had to do this myself and the relay for my TRANE unit was less than $15.
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