Contactor Testing and When to Replace

The short version

A contactor is a switch. The thermostat sends 24 volts to a coil, the coil pulls a set of contacts closed, and line voltage reaches the compressor and condenser fan. When the contacts pit, weld, or the coil burns out, the outdoor unit either will not start or will not stop.

Testing takes a multimeter and about five minutes. Measure coil resistance with power off and coil voltage with the control circuit on. Check the contacts for continuity both pulled in and released. Those four readings sort most faults into the contactor or the control circuit and narrow what to replace, though a contactor that passes them can still run hot under load.

The customer pays for a bad contactor left in service. Welded contacts and chattering both stress the compressor motor, and either can turn a contactor job into a compressor job.

What a contactor does

A contactor is an electrically operated switch that controls line-voltage loads in HVAC equipment. In a residential condensing unit, the contactor switches 240 VAC to the compressor motor and the condenser fan motor. The type used in residential and light commercial equipment is a definite purpose contactor, designed for the predictable duty cycles of heating and cooling systems and covered by UL 60947-4-1. UL closed UL 508 to new and alternate construction investigations of magnetic motor controllers after January 26, 2017.

The low-voltage side is the coil. When the thermostat calls for cooling, 24 VAC from the control transformer energizes the coil, and the magnetic field pulls a plunger down. The plunger carries the contacts, which are the high-voltage side. When the contacts close, line voltage passes through to the compressor and fan.

The 24-volt control circuit runs from the transformer secondary through the thermostat and any safety controls to the contactor coil. If any safety switch in that loop is open, 24 volts never reaches the coil and the contactor stays open. That is the system working as designed, not a contactor fault. Checking whether voltage reaches the coil is the branch point. No voltage points upstream. Correct voltage with no pull-in points at the contactor.

A residential contactor is typically rated at 30 or 40 full load amps (FLA). The FLA rating governs motor loads. The contactor label also shows a resistive amperage rating, often marked “Res,” that is higher than the FLA number. Size a replacement by the FLA rating, not the resistive one, because a motor is an inductive load and the inductive rating applies.

The contacts carry the full starting inrush of the compressor every time they close. That inrush, called locked rotor amps (LRA), is several times the running amperage. Each closure produces a small arc as the contacts meet, and over thousands of cycles the arc erodes the contact surfaces. That erosion is normal wear, and it is why contactors are consumable parts with a limited service life.

How to test a contactor

Testing a contactor takes a multimeter and two stages: a de-energized check of the coil and contacts, and an energized check of coil voltage. The energized check involves 24 volts on the coil side, but line voltage is present at the contact terminals in the same enclosure. Run the de-energized tests first because they are safer and they catch most contactor faults without powering the system on.

Isolate the equipment before you open the panel. The National Electrical Code (NEC), in Article 440, requires a disconnecting means within sight of the equipment and readily accessible from it. Open it, then kill the 24-volt control power too, because the outdoor disconnect does not open the control circuit.

The Occupational Safety and Health Administration (OSHA), in 1910.333(b)(2)(iii)(A), requires a lock and a tag on each disconnecting means. A tag alone is allowed only where a lock cannot be applied and a second measure gives equivalent protection. OSHA 1910.333(b)(1) treats equipment de-energized but not locked out or tagged as energized. Then verify dead at every part you will be exposed to, which OSHA 1910.333(b)(2)(iv)(B) requires, not just the line-side lugs. That means line, load, and coil terminals, plus the capacitor terminals and anything else live in the compartment.

De-energized tests. With every source isolated, locked and tagged, and verified dead:

Set the meter to resistance. Pull the two wires off the coil terminals first, because the rest of the control circuit can skew the reading. That order comes from AC Service Tech. Place the probes across the terminals. The meter should show resistance on a good coil. According to AC Service Tech, an open-line reading (OL on most meters) means the coil is burned out. A reading of zero ohms means the coil is shorted. In either case, replace the contactor.
Pull the power wires off the contacts too, because current can reach the meter through wiring left connected. That is how AC Service Tech runs this test. Without pressing the plunger in, place the probes across one set of line-to-load terminals (L1 to T1). The meter should read OL, meaning the contacts are open. If the meter reads zero ohms or near zero, the contacts are welded shut. AC Service Tech describes this as contacts welded together due to high amperage melting the contacts together. Repeat on every switched pole, counting them first: AC Service Tech notes HVAC contactors come as single, double, or triple pole, and a single-pole unit’s shunted leg reads continuity by design.
Press the contactor plunger in by hand. The meter should now read zero ohms across the same terminals. If you see OL or high resistance with the plunger pressed in, the contacts are not making a connection. Check the contact faces for pitting, burning, or debris.

Energized coil voltage test. OSHA 1910.334(c) allows only a qualified person to test energized equipment, with a meter and leads rated for the circuit. Restore power to the line and control circuits. Set the meter to AC voltage. With the thermostat calling for cooling, place the probes across the coil terminals. According to AC Service Tech, the reading should be between 24 and 29.5 volts. Below that range, the coil may not pull in fully, which causes chattering. If you measure zero volts with a call for cooling, the 24-volt control circuit is open upstream of the contactor. The problem is in the transformer, thermostat wiring, or a safety control, not the contactor.

What the readings tell you

Test	Good reading	Bad reading	What it means
Coil resistance (power off)	Resistance in ohms	OL	Coil is open, replace the contactor
Coil resistance (power off)	Resistance in ohms	0 ohms	Coil is shorted, replace the contactor
Coil voltage (call for cooling)	24 to 29.5 VAC	Below 20.4 VAC (85 percent of rating)	Low voltage from transformer, wiring, or thermostat
Coil voltage (call for cooling)	24 to 29.5 VAC	0 VAC	Control circuit is open upstream of the contactor
Contact continuity, isolated switched pole (plunger released)	OL	0 ohms	Contacts are welded or stuck closed
Contact continuity, isolated switched pole (plunger pressed in)	0 ohms	OL or high resistance	Contacts are pitted, burned, or blocked by debris

A contactor that passes all four tests has cleared the gross faults, not every fault: Schneider Electric’s position is that contactors should be tested under load, not with a digital meter, even in-service ones.

Two results call for a closer look before you condemn the part. A coil that reads within range on resistance but will not pull in may be receiving low voltage rather than failing mechanically. Measure coil voltage before replacing a contactor that tests good on resistance but chatters or will not close. AC Service Tech notes that a corroded electrical connection at the spade terminal can drop the voltage reaching the coil enough to prevent closure.

Contacts that show high resistance with the plunger pressed in are not always pitted. AC Service Tech identifies insects as a cause: ants and spiders can lodge between the contact faces and prevent a clean connection. Inspect the contact faces before you order a part.

When to replace

Beyond the meter readings, four conditions call for replacement.

Pitted or blackened contacts. Every closure erodes the contact surface slightly. Light pitting is normal wear. When the pits are deep enough that the contacts no longer make flat contact, resistance across the connection rises and the contacts heat under load. The heat accelerates the damage. Replace the contactor when the pitting is deep enough to hold heat, rather than waiting for welding, which has its own causes.

Welded contacts. If the compressor runs after the thermostat satisfies and the disconnect is the only way to stop it, suspect welded contacts. A welded contactor is replaced immediately because the system has lost its normal off cycle.

Chattering. A contactor that rapidly opens and closes is not pulling in or holding. Buzzing alone is weaker evidence, because it has other causes. Chattering causes arcing at every partial closure, which pits the contacts faster than normal operation would. Check coil voltage first. If voltage is in range and it still chatters, look past the coil: Schneider Electric lists a broken shading coil, dirt on the magnet pole faces, armature misalignment, binding parts, and the wrong coil among the causes.

Melted or cracked housing. Heat from pitted contacts or from a short travels into the plastic body. A contactor with visible heat damage to the housing is replaced regardless of what the meter reads, because the structure holding the contacts in alignment is compromised.

Sizing the replacement. Match three specifications from the original: coil voltage, pole count, and FLA rating. The coil voltage is printed on the contactor body. Most residential systems use a 24 VAC coil, but some equipment uses a 240 VAC coil. Installing a 24-volt coil on a 240-volt circuit burns the coil immediately, and a 240-volt coil on a 24-volt circuit will not pull in. Match the pole count to the original, and make sure the contactor’s FLA rating meets or exceeds the compressor’s rated load amps.

HVAC School, writing about what else to replace when you change a compressor, says that a contactor showing signs of wear is good practice to replace, especially in three-phase units where a single contact failure can cause single phasing.

Single-pole and two-pole contactors

Residential condensing units use either a single-pole or a two-pole contactor. A single-pole contactor switches one leg of the 240-volt circuit. The other leg passes through a solid copper bar called a shunt, which stays connected whether the contactor is energized or not. A two-pole contactor switches both legs.

The shunt exists for a reason. In some equipment, the unswitched leg powers the crankcase heater. A crankcase heater keeps the oil warm so refrigerant vapor does not migrate to the colder compressor during the off cycle and dissolve into the oil. HVAC School explains the circuit: in certain configurations the crankcase heater back-feeds through the compressor winding on the unswitched leg, and it functions only when the contactor is open.

If you replace a single-pole contactor with a two-pole unit wire for wire, the crankcase heater stops working because both legs now open when the contactor de-energizes. HVAC School states this directly: if you replace this contactor wire for wire with a two-pole contactor, the crankcase heater will never work. Check the wiring diagram before ordering the replacement. If the equipment requires a shunt circuit, either match the original pole count or rewire according to the manufacturer’s diagram.

Common mistakes

Lock and tag both the line-voltage disconnect and the control power, then confirm dead at the line, load, coil, and capacitor terminals before touching anything inside
Test coil voltage before condemning a contactor that chatters, because low voltage from the control circuit produces the same symptom
Check the wiring diagram before replacing a single-pole contactor with a two-pole, because the crankcase heater circuit may depend on the shunt
Match the replacement contactor’s FLA rating to the load rather than using the resistive amp rating on the label
Inspect the contact faces for insects and debris before ordering a replacement based on a high-resistance reading

Don’t

Test contact continuity with the disconnect closed, because line voltage is present at the contacts
Assume a chattering contactor is failing when the coil is receiving low voltage from a corroded terminal or a weak transformer
Ignore severely pitted contacts because the system still runs, since the added resistance heats the contacts under load
File or sand pitted contacts to extend their life, because filing removes the silver contact surface, which is what Schneider Electric gives as the reason not to do it

Frequently asked questions

How do I know if my contactor is bad?

A contactor is bad when it fails one of the four readings: coil resistance, coil voltage, or contact continuity with the plunger released or pressed in. Visible signs include pitted or blackened contact faces, a melted housing, or contacts that are stuck together.

Can I clean the contacts instead of replacing the contactor?

Filing or sanding the contacts is not a reliable repair. The contact surfaces on a definite purpose contactor are a layer of contact material over a base metal. Schneider Electric’s reason for not filing is that filing removes that silver surface. HVAC School, in its guidance on compressor replacement, treats a worn contactor as a candidate for replacement rather than repair.

How long does a contactor last?

A contactor’s service life depends on how many times it cycles and how much current it interrupts at each cycle. In residential air conditioning, contactors typically last five to ten years under normal conditions. High starting loads, low line voltage, and outdoor exposure to moisture and insects shorten that life. Schneider Electric publishes a durability figure of one million electrical operations for a NEMA size 1 contactor, which is a rated figure for a different class of device and not a field expectation for a residential unit.

What causes a contactor to weld shut?

High current at the contact surfaces melts the contact material, and the contacts fuse together when they cool. The most common causes are a contactor rated below the compressor’s rated load amps (RLA), a compressor drawing more amperage than its rating, and low line voltage, which increases the current the compressor motor draws. AC Service Tech describes welded contacts as resulting from high amperage melting the contacts together.

Will a bad contactor damage the compressor?

A contactor that chatters connects and disconnects the compressor over and over, which stresses the windings and can shorten the motor’s life. A contactor with welded contacts keeps the compressor running through every off cycle, which can overheat the motor. Either condition, left in place, makes a compressor failure more likely.

Glossary

Coil: the electromagnetic winding inside the contactor, wound for the control voltage in use, that creates the magnetic field pulling the contacts closed when energized.
Contactor: an electrically operated switch that uses a control-voltage coil to open and close line-voltage contacts, controlling power to a compressor and condenser fan motor.
Definite purpose (DP) contactor: a contactor designed for HVAC equipment duty cycles, covered by UL 60947-4-1. UL closed UL 508 to new and alternate construction investigations of magnetic motor controllers after January 26, 2017.
FLA (full load amps): the maximum current a motor draws under rated load at rated voltage. The contactor’s FLA rating must meet or exceed the load’s FLA.
LRA (locked rotor amps): the current a motor draws at the instant of startup before it begins to turn, several times higher than FLA.
NEC: National Electrical Code. Article 440 covers air conditioning and refrigeration equipment, including disconnect requirements.
RLA (rated load amps): the figure a compressor manufacturer publishes for its compressor in place of FLA. Copeland describes RLA as the reference current used in contactor and wire selection.
Shunt: a solid copper bar on a single-pole contactor that provides an unswitched path for one leg of the circuit, allowing components like crankcase heaters to remain powered when the contactor is open.
Single phasing: a condition in a three-phase system where one phase is lost, causing the motor to run on two phases and potentially overheat.

Drafted with AI assistance and reviewed by the author.

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