Contactor Failure: When the Outdoor Unit Will Not Start, or Will Not Stop

Check these first

A contactor is a heavy-duty relay. The thermostat sends it a small signal, and it closes a set of contacts that send full power to the compressor and fan. It is a simple part with a simple job, and the ways it fails are simple too, which is why it is worth ruling out the things that imitate it first.

• Thermostat. If the outdoor unit will not start, confirm the thermostat is calling: Cool mode, setpoint below the room. If the unit will not stop, set the thermostat to Off and wait two minutes. If it keeps running, that is your answer.
• Filter. Unrelated to the contactor, but a clogged one makes everything else worse. Check it.
• Breaker. Both the indoor and outdoor breakers. Reset a tripped one once.
• Listen. When the thermostat calls, do you hear a firm click at the outdoor unit, then the fan? No click at all points here or at the thermostat wire. A rapid clicking or buzzing, like a relay in a lightning storm, is chattering.
• Debris. Clear what is around the base. Do not open the panel to clear what is inside it.
• Shut it off if it will not stop. If the outdoor unit runs with the thermostat off, turn the outdoor breaker off. That is the one time we will tell you to flip a breaker for something other than a reset.

What it usually means

Will not start

The contacts are pitted, burned, or blocked, and the circuit does not close when the coil pulls in. You may hear the click and nothing after it. Or the coil itself has failed and there is no click. Or a very small insect has made the gap its home, of which more below.

Will not stop

The opposite failure and the more expensive one. Contacts that have arced enough times weld themselves closed. The thermostat drops its signal, the coil releases, and the contacts stay put. The outdoor unit runs until the breaker is thrown. Left alone, the compressor runs on a cold coil, the evaporator ices, and the bill reflects a unit that never turned off during an SRP or APS peak window. A system that ignores its thermostat is a system that needs its outdoor breaker off tonight and a technician tomorrow.

Chattering

A contactor that clicks rapidly is trying to pull in and failing to hold. Causes, in the usual order: a weak thermostat signal from a corroded wire or a failing transformer, low voltage at the unit during a monsoon-season brownout, or grit on the contact surfaces that keeps them from seating. Chattering is hard on everything downstream, because each half-close sends a pulse of current to a compressor that did not ask for it.

Why ants and debris matter

This is not a Phoenix myth. Small ants are drawn to the warmth and the faint electrical field inside the contactor and crawl into the contact gap, where they are electrocuted on the next cycle, leaving a body that keeps the contacts apart or bridges them. Enough of them and the contactor is a fossil bed. Haboob dust does the same thing more slowly, and a gecko will occasionally manage it in one attempt. Contactors on ground-level units near landscaping see the most of it; rooftop units get the dust instead. None of this is the homeowner's fault. It is simply the reason the panel gets opened and inspected during spring maintenance, not just in the emergency.

Repair scope

A contactor replacement is one of the simpler electrical repairs in the outdoor unit and the one we least want to leave to a homeowner, because the panel it lives behind carries line voltage and shares space with the capacitor. The technician disconnects power, confirms it is off with a meter, replaces the contactor with one matched to the coil voltage and amperage of the original, tightens the lugs, and then does the part that matters: checks why it failed. Pitted contacts on a young unit suggest voltage problems or a compressor drawing more than it should. A welded contactor on an older unit suggests the compressor has been hard-starting for a while. That inspection is included in the visit, and the flat-rate quote is written before the panel comes off. The electrical repairs page covers the neighboring parts.

On a heat pump, the same contactor cycles in winter too, so it logs roughly twice the operations of a cooling-only unit and wears accordingly. That is worth knowing if your system is a heat pump on a rooftop in Peoria or Glendale that has never had its panel opened.

A contactor by itself is never a reason to replace a system. A contactor that has welded twice, on a fifteen-year-old unit whose compressor groans on every start, is a reason to read the repair-versus-replace guide before the next summer. If that is where it lands, financing on approved credit is available and the estimate comes first.

Stop and call if

• The outdoor unit keeps running with the thermostat set to Off. Turn the outdoor breaker off and call.
• You hear rapid clicking or buzzing from the outdoor unit. Set the thermostat to Off.
• The breaker trips a second time after one reset.
• You smell scorched insulation or see discoloration on the outdoor cabinet near the panel.
• The house is above 85 and someone in it is elderly, very young, pregnant, or managing a chronic condition. Cooling them comes before diagnosing the unit. Move them somewhere cool and call.

A web page cannot tell a welded contactor from a shorted thermostat wire, and a homeowner should not open the panel to find out. A technician with a meter can, quickly. Our repair trucks carry common contactors, and the fix, when it is the whole story, is short.