What type of control display do you have?
Does the fan stop running when it reaches setpoint?
Also, it may be that you have the system in auto mode.
It will run in heat or cool relative to the setpoint vs inside temp as read by the sensor.
The sensor....that's where you start.
If you have a remote
sensor then check
that it's not touching anything and its in the return air path.
Some units will also have a sensor in the display itself, which the system will default to when the remote
sensor is unplugged from the control board.
Think about the sensor you have and how its reacting to the temps.
Consider the system blind, it only knows what IT'S sensor is telling it.