Diagnosing thermostat issues was the primary task during this HVAC service call. I started by checking thermostat wiring connections at both the thermostat and indoor unit for any loose terminals. Using a multimeter, I verified the thermostat was receiving 24V power from the transformer. To test thermostat operation, I bypassed the unit directly from the control board, confirming the system operated correctly. This isolated the malfunction to the thermostat itself. After consulting the thermostat's technical data, I recalibrated the thermistor sensor and updated the firmware to restore proper temperature sensing and system control functionality.