The outdoor unit hums but won’t start. Or it won’t shut off even when you set the thermostat to OFF. Or you hear a sustained buzzing from the condenser cabinet. In all these cases, the contactor is the likely culprit.
The contactor is the second most replaced component in a residential AC system after the capacitor. It’s a $15–35 part, and replacement follows the same safety protocol as the capacitor: discharge first, then swap.
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What the AC contactor does
The contactor is a high-amperage relay — an electromechanical switch that connects 240V line voltage to the compressor and condenser fan when the thermostat calls for cooling.
When the thermostat calls for cooling:
- The control board sends 24V AC to the contactor coil.
- The coil generates a magnetic field that pulls a plunger (the armature) down.
- The plunger closes a set of high-voltage contacts, completing the 240V circuit to the compressor and fan.
- When the thermostat is satisfied, the 24V signal drops, the coil de-energizes, and a spring pushes the plunger back up, opening the contacts.
The contacts open and close thousands of times over the life of the unit. Each closing pulls several hundred amps of inrush current; each opening creates a small arc as the contacts separate. Over time, the contact faces pit and erode. When they’re too pitted, they resist current flow (causing heat), weld shut, or fail to close properly.
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Symptoms of a failing or failed AC contactor
Outdoor unit buzzes loudly but doesn’t start. If the contactor coil is pulling the plunger down (you can hear or feel the buzz of the electromagnet) but the contacts aren’t making solid connection, the compressor gets insufficient voltage. This usually means badly pitted or burned contact surfaces.
Outdoor unit won’t start at all — no sound, no activity. If the contacts are stuck open, the 240V never reaches the compressor. Alternatively, if the coil itself has failed (burned out), the plunger won’t pull down, and the contacts stay open.
Outdoor unit won’t shut off with thermostat set to OFF. The contacts are welded or stuck closed. The compressor and fan run regardless of the thermostat because 240V line voltage is always passing through. This is a safety situation — the unit runs without control. Turn off the breaker for the AC until the contactor is replaced.
Buzzing, chattering, or clicking from outdoor unit during normal operation. A loose plunger, a vibrating contact, or low voltage to the coil can cause the contactor to chatter — rapidly opening and closing — which sounds like rapid clicking or buzzing from the cabinet.
Visible burn marks or pitting on contact faces. With power off and the capacitor discharged, visually inspect the contacts. Significant pitting (contact surface looks pockmarked, eroded, or shows burn marks) means the contactor is near end of life even if it’s still making electrical contact.
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⚠️ Safety: line voltage and capacitor discharge
Line-voltage work requires two steps before touching anything inside the condenser cabinet:
- Turn off the outdoor disconnect (the gray box on the wall next to the outdoor unit — pull out the block or flip the switch).
- Turn off the circuit breaker at the main panel for the AC.
- Wait 30 seconds, then discharge the capacitor. See HVAC Capacitor: How to Test and Replace for the discharge procedure. The contactor is adjacent to the capacitor, and you’ll be near both.
- Verify zero voltage across the load-side terminals of the contactor with a multimeter set to 240V AC before touching any wires.
The line-side terminals of the contactor (where the wires from the disconnect come in) remain energized until the disconnect AND the breaker are both off. Don’t rely on only one.
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How to test a contactor
Visual inspection (first):
- Remove the lower panel or side access panel on the outdoor unit (usually 1/4-inch screws).
- Locate the contactor — a rectangular device roughly 2×3 inches, with large wires connected to both top (line) and bottom (load) terminals.
- Inspect the contact faces: a small window or opening usually lets you see the contact surface. Look for pitting, erosion, or carbon buildup.
- Inspect the coil wires: look for cracked insulation, burned wiring at the coil terminals.
Electrical test (after discharge):
- Set multimeter to continuity or resistance.
- Check across the contact pairs with NO voltage applied and the plunger in the UP position (de-energized). You should read OPEN (no continuity). Continuity when de-energized = contacts stuck closed.
- Manually depress the contactor plunger by hand (with power off and capacitor discharged). Check continuity across contacts — you should now read CLOSED (continuity). No continuity when manually closed = contacts too pitted or failed.
Coil test:
- Disconnect the 24V wires from the coil terminals.
- Test resistance across the coil terminals. A working coil reads approximately 10–60 ohms depending on the contactor — the exact value varies by model, but an open reading (∞ or OL) indicates a burned coil.
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Replacing the contactor
What to buy: a replacement contactor must match the original in:
- Pole count (single-pole or double-pole) — double-pole is more common and preferred (breaks both legs of the 240V circuit)
- Coil voltage — almost universally 24V AC for residential units
- Contact amperage rating — typically 30A or 40A; match or exceed the original rating
Take the old contactor to an HVAC supply house or photograph its label. Replacement brands (Siemens, Packard, Honeywell, Schneider Electric) are interchangeable if specs match. Cost: $15–40 at supply houses; higher on Amazon.
Steps:
- Turn off power (disconnect + breaker). Discharge capacitor. Verify zero voltage.
- Photograph the wiring on the contactor before removing anything — line-side, load-side, and coil wires.
- Remove wires one at a time, noting which terminal each connects to. Contactor wires use spade connectors or ring terminals — pull or unscrew.
- Remove the mounting screws (usually 2 screws into the unit’s mounting bracket).
- Install the new contactor in the same orientation.
- Reconnect wires per your photo. Double-check that line wires go to line terminals (top) and load wires to load terminals (bottom). Swapping them is a common mistake that prevents operation.
- Restore power and test. Call for cooling. The outdoor unit should start within 1–2 minutes of the thermostat call.
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Shop by your contactor’s rating
Match three things to the old unit: pole count, contact amperage, and a 24V coil (residential condensers are 24V).
Supply houses are usually cheaper than Amazon on this part, as the cost section above says. These links are for when you need it today and the counter is closed.
If you buy through these links we may earn an affiliate commission, at no extra cost to you. As an Amazon Associate we earn from qualifying purchases.
Cost comparison
| Approach | Part | Labor | Total |
|---|---|---|---|
| DIY | $15–40 | — | $15–40 |
| HVAC service call | $20–50 | $130–250 | $150–300 |
The contactor is the other great DIY AC repair alongside the capacitor. The repair takes 20–30 minutes. The only barrier is comfort with line-voltage safety and the discharge procedure.
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When to call a pro
- You can’t confirm power is off at both the disconnect and breaker, or the capacitor discharge makes you uncertain.
- The outdoor unit still won’t start after replacing the contactor. The compressor itself may be failed or locked — check capacitor first if you haven’t already, then call a tech for compressor diagnosis.
- The contactor replacement fixes the start, but the unit runs briefly and shuts down. Low refrigerant, a refrigerant leak, or a high-pressure fault — these require a tech with gauges and EPA 608 certification.
- Wiring damage around the contactor — burned insulation, scorch marks, or melted wire connectors indicate a serious overcurrent event; have the full circuit inspected before restarting.
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FAQ
What’s the difference between a contactor and a relay? Both are electrically operated switches, but a contactor is rated for much higher amperage (20–40A for HVAC vs. a few amps for a relay) and is built to handle the repeated heavy inrush current of compressor startups. A relay is a contactor designed for lighter loads.
My outdoor unit keeps running with the thermostat off. Is this dangerous? Yes — the contactor is stuck closed. The compressor is running without thermostat control, which can cause refrigerant migration, compressor damage, or freeze-up. Turn off the disconnect switch next to the unit until the contactor is replaced.
Can I temporarily bypass a stuck-open contactor to get the AC running? Technically possible by manually holding the plunger depressed, but this is not safe as a running fix. If you’re comfortable with the safety steps, replace the contactor properly. In an emergency, a tech can often do a same-day service call.
How long do AC contactors last? Typically 5–15 years depending on run hours and number of starts per day. Contactors in climates with long cooling seasons and many on/off cycles wear faster. If your unit cycles frequently (short-cycling), the contactor wears faster.
Can a bad contactor damage my compressor? Yes — indirectly. A chattering contactor (rapidly opening and closing) delivers erratic voltage to the compressor, which causes heat and accelerates winding wear. A stuck-closed contactor runs the compressor uncontrolled. Replace a failing contactor promptly.
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Related guides
- AC Not Cooling: Diagnostic Order — if the contactor is fine but the AC still doesn’t cool, continue with the full diagnostic
- AC Outdoor Unit Not Running: What to Check — contactor failure is the second check (after capacitor) when nothing at the outdoor unit starts
- HVAC Capacitor: Symptoms and Replacement — the most common outdoor-unit repair, often done at the same service call
For a full repair-cost table by component (capacitor, contactor, fan motor, refrigerant, board), see AC repair cost by component.
