Switching & protectionRebuild shelfTest and identify

Power relay / contactor

Also called mains relay, T9 relay, HVAC relay, contactor.

Contacts that can actually switch a heater, a motor, or a pump. Found on appliance boards, HVAC, window AC, UPS, and garage-door openers. Coil voltage and contact rating are both mandatory reading.

If you are new to this category

A switch opens or closes a circuit. Some switches are for your finger. Some open when they get too hot and stay open. Those do not reset. A relay is a switch that a small current turns on so it can control a larger current. Use the rating printed on the part.

Click it and listen. Then use a meter on continuity, which beeps when the contacts touch. Do not guess that a small switch can run a motor.

The three legs of a snap switch

Common is the moving contact. NO means normally open: that leg connects only while you press. NC means normally closed: that leg is connected until you press. Click the button while the meter is on continuity and write down which beep matches which leg.

Mains voltage. Line voltage, and the DC bus just after the rectifier, both kill. A disconnected cord is not a discharged supply.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Coil	5, 12, 24 VDC, or 24 VAC on HVAC. A 24 VAC coil on DC will not behave.
Contacts	10–30 A is common on appliance power relays. Motor loads want a motor rating, not just a resistive one.

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

Letting a small board switch a real mains load
HVAC-style control

Leave this one when

The contact rating is worn off. Do not put an unknown relay on a heater.

Buying a new one is the better idea when

A new contactor for anything that runs unattended.

How to tell if this one still works

Coil resistance and a click at rated voltage. Contact resistance should be near zero. Pitted contacts that still beep may weld shut next time.

Tools you will want: Meter.

Where people actually find it

7 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Window air conditioner	Worth it	1	usually	Contactor or relay.
Uninterruptible power supply	Worth it	1–3	usually	Transfer relays.
Garage door opener	Worth it	1	usually	The logic board.
Washing machine	Bonus	several	usually	Control board.
Dishwasher	Bonus	several	usually	Control board.
Paper shredder	Bonus	1	sometimes	Auto-reverse boards.
End-of-life car	Leave it	modules	always	Airbag control, pretensioners.

Worth pullingWindow air conditioner1 · usually
In this particular machine
Switches the compressor.
Where to look
Contactor or relay.
How to get it out
Read the coil voltage, often 24 VAC or mains.
The condition to expect
Pitted contacts if the unit short-cycled.
Worth pullingUninterruptible power supply1–3 · usually
In this particular machine
Mains contacts.
Where to look
Transfer relays.
How to get it out
Read the rating.
The condition to expect
Good.
Worth pullingGarage door opener1 · usually
In this particular machine
Motor reversing.
Where to look
The logic board.
How to get it out
Read it.
The condition to expect
Good.
Bonus, if you are already in thereWashing machineseveral · usually
In this particular machine
Motor and heater relays.
Where to look
Control board.
How to get it out
Read the contact rating.
The condition to expect
Pitted contacts are a maybe.
Bonus, if you are already in thereDishwasherseveral · usually
In this particular machine
Heater and pump relays.
Where to look
Control board.
How to get it out
Read the rating.
The condition to expect
Mixed.
Bonus, if you are already in therePaper shredder1 · sometimes
In this particular machine
Current-sense reverse.
Where to look
Auto-reverse boards.
How to get it out
Only with the board.
The condition to expect
Mixed.
Leave it — present, not yours to reuseEnd-of-life carmodules · always
In this particular machine
Yellow connectors. Pyrotechnics.
Where to look
Airbag control, pretensioners.
How to get it out
Do not take them.
The condition to expect
Not parts.