Contactor failure occurs when the electromagnetic switch fails to close properly, preventing electrical current from reaching the compressor and outdoor fan motor. Contactors use a control voltage signal to engage a solenoid coil, which closes heavy-duty contacts carrying 15 to 60 amperes of current. When contacts burn, pit, or weld together, the system cannot start or runs continuously without thermostat control.
Failure Mechanisms
Repeated electrical arcing across contacts causes pitting and erosion, reducing contact surface area and increasing resistance. Voltage spikes, moisture intrusion, and high ambient temperatures above 95 degrees Fahrenheit accelerate contact degradation. Stuck contactors fail to release even when thermostat calls for shutdown, running compressors continuously and wasting energy.
Diagnosis and Service
Technicians verify contactor function by measuring control voltage at the coil (typically 24 volts) and checking for audible clicks during thermostat cycling. Visual inspection reveals burned contacts appearing black or pitted. Replacement involves de-energizing circuits and installing new contactors matching original voltage and amperage ratings.
Lifespan and Maintenance
Quality contactors last 10 to 15 years with proper voltage regulation. Annual inspections identifying contact erosion enable preventive replacement before system failure.