To diagnose electrical control failures using multimeter measurements, start by measuring voltage and current at various points in the system, such as at the thermostat, relays, and contactors. Compare these measurements to the manufacturer’s specifications and system design documents. Look for signs of voltage drops, current imbalances, or unusual resistance readings, which can indicate faulty components or wiring issues. Additionally, use the multimeter to measure capacitance, resistance, and continuity to identify faulty capacitors, relays, or contactors.