Furnace Contactor and Relay: What They Do and When to Replace Them

Contactors and relays are electrically operated switches that control the flow of power to major HVAC components. They're relatively inexpensive but critically important — a failed contactor or relay is a common cause of AC and furnace problems that often gets misdiagnosed. Here's what you need to know.

Contactors: The High-Power Switch

What a Contactor Does

The contactor is found in your outdoor AC or heat pump unit. It's a heavy-duty electrical switch that controls power to the compressor and condenser fan motor — both high-current loads (typically 30–60 amps combined). The contactor is controlled by a small 24V signal from the thermostat/control board, which energizes the contactor coil, pulling the contact arms together and completing the 240V circuit to the compressor.

Every time your AC starts, the contactor closes. Every time it stops, the contactor opens. Over a Minnesota cooling season with thousands of cycles, contactors wear out.

Signs of a Failing Contactor

• AC won't start — outdoor unit doesn't respond when AC is called. Check if you can hear the contactor click when cooling is called (with power on, listen near the unit — do not touch electrical components)
• AC runs constantly — contactor contacts have welded together (fused closed) and can't open. The compressor runs even when it shouldn't.
• Buzzing or chattering sound from outdoor unit — contactor coil is trying to pull in but can't fully close, causing rapid cycling of the contacts
• Pitted or burned contacts — visible during inspection when power is off. Severe pitting increases contact resistance and causes overheating

Contactor Replacement

Contactors are typically $15–$40 for the part and $80–$150 labor to replace. This is one of the most common AC repairs — budget for it on units over 8 years old. Always replace with a contactor of matching amp rating and coil voltage (typically 24V coil, 30–40 amp contacts for residential single-phase AC).

Relays: The Low-Power Switch

What a Relay Does

Relays in HVAC systems are smaller switches than contactors, controlling lower-current loads — blower motors, inducer motors, control circuits. The modern Goodman control board contains relay circuits integrated into the board itself, which is why control board failures often manifest as specific component failures (blower won't start, inducer won't run) rather than complete system shutdown.

Types of Relays in HVAC Systems

• Fan relay: Controls blower motor operation — can be discrete or integrated into control board
• Time-delay relay: Controls blower on/off timing relative to heat call
• Transformer relay: Protects the 24V control transformer from overload
• Heat relay (electric furnaces): Energizes individual heating elements in sequence

Signs of a Failed Relay

• Specific component doesn't operate (blower runs but inducer doesn't, or vice versa)
• Intermittent operation — component sometimes works, sometimes doesn't
• Chattering or buzzing from the control board area

On modern Goodman furnaces with integrated control boards, an individual relay failure usually means replacing the entire control board — discrete relay replacement isn't practical on circuit board-integrated designs.

Capacitors vs. Contactors: Don't Confuse Them

Both capacitors and contactors are commonly involved in AC startup failures. They're different components:

• Capacitor: Stores electrical energy to provide startup boost to motors. Fails with age, causing humming without starting. See our capacitor guide.
• Contactor: A mechanical switch. Fails due to worn contacts or failed coil. Prevents power from reaching the compressor at all.

A complete diagnostic tests both. Replacing one when the other is the actual problem wastes money.

DIY vs. Professional Contactor Replacement

Contactor replacement involves working near 240V components. While mechanically straightforward, the risk of electrical shock makes this a job for technicians or technically experienced homeowners who take appropriate precautions. Always:

• Disconnect power at the breaker AND the local disconnect before touching anything inside the unit
• Wait 5+ minutes after power disconnect before touching capacitors (they store charge)
• Photograph existing wiring before disconnecting
• Use insulated tools

Replacing Electrical Components on Aging Equipment

If your Goodman furnace or AC is under warranty and a contactor or relay fails, the part may be covered under the 10-year parts warranty — check your registration status. On equipment over 10 years old, weigh repair cost against a factory-direct replacement. Furnace Direct can get you a new Goodman system at wholesale pricing.

Related: Capacitor Guide | Control Board Guide | Compressor Guide