How do I troubleshoot defrost cycle failure to engage in a heat pump system?

To troubleshoot defrost cycle failure to engage, first verify that the system’s defrost control components, such as sensors and relays, are functioning correctly. Check the outdoor coil temperature sensor for accuracy and ensure that it is not faulty or damaged. Inspect the defrost timer and verify that it is set correctly. If the issue persists, check the system’s refrigerant charge and ensure that it is at the recommended level. Finally, inspect the outdoor coils for excessive ice buildup or debris accumulation, which can prevent the defrost cycle from engaging.