The most common repair on a boiler often involves issues with the thermostat or temperature controls. Problems may stem from faulty sensors, wiring issues, or calibration problems, leading to inconsistent heating or temperature fluctuations. Additionally, sediment buildup or mineral deposits in the system can affect thermostat performance. Regular maintenance, including thermostat calibration, sensor checks, and system flushing, can help prevent and address these common boiler issues, ensuring efficient and reliable heating operation.