What should always be checked if a furnace is cycling on and off too frequently?

In the scenario where a furnace is cycling on and off too frequently, it is crucial to consider multiple factors that could contribute to this issue. Each choice represents an important aspect of a furnace's operation that should be inspected for proper functionality.

Thermostat calibration plays a significant role in regulating the temperature effectively. If the thermostat is not accurate or properly set, it can cause the furnace to run longer or shorter than necessary, leading to frequent cycling.

The condition of the air filter is also vital; a dirty or clogged filter can restrict airflow. This can cause the furnace to overheat and subsequently shut off as a safety precaution. Ensuring the air filter is clean allows for proper airflow and efficient operation.

Lastly, examining the electrical connections is essential as loose or faulty connections can lead to intermittent operation. If the furnace is not receiving stable power, it may turn on and off erratically.

By checking all these elements—thermostat calibration, air filter condition, and electrical connections—you address the potential causes of the cycling issue comprehensively. Thus, the correct answer encompasses the need to investigate all these areas to diagnose and resolve the problem effectively.