Why is my AC set at 74 but reads 78?

When your air conditioning thermostat is set to 74 degrees Fahrenheit but the screen or indoor reading shows 78 degrees, it indicates that your cooling system is struggling to remove heat fast enough to meet your target. Common causes include a severely clogged air filter that restricts airflow across the indoor coil, dirty outdoor condenser coils that block heat rejection, or low refrigerant levels from a leak that reduce cooling capacity. Additionally, extreme outdoor heat waves, poor home insulation, or drafty windows can introduce more heat into the home than the AC can handle. You can troubleshoot by replacing dirty air filters and clearing debris from your outdoor condensing unit. You can verify that the issue is resolved when the system's supply air feels crisp and indoor temperature readings begin dropping steadily toward your 74-degree setpoint.