What a relay does and why it fails
A relay is an electromagnetic switch inside your car, appliance, or equipment that uses a small electrical signal to turn a larger electrical circuit on or off. When you press a button or a sensor detects something, it sends power to the relay's coil. That coil creates a magnetic field, which pulls a metal contact closed and completes a second circuit — often one that draws much more power than the button itself could handle.
Relays fail in two main ways. The coil inside can burn out or corrode, so it no longer responds to the signal. Or the metal contacts inside can get stuck, corroded, or worn, so they no longer make a clean connection even when the coil pulls them closed. A multimeter can test both of these problems without removing the relay from the circuit, though you will get clearer results if you pull it out first.
Key Takeaways
- A relay has two separate circuits: a low-power coil circuit and a high-power contact circuit, and you test each one separately with a multimeter.
- Set your multimeter to resistance (ohms) to check whether the coil is burned out, and to continuity to check whether the contacts are stuck or corroded.
- A good relay coil usually reads between 50 and 200 ohms; a dead coil reads infinite or very high resistance.
- The contacts should show zero or near-zero ohms when the relay is energized, and infinite resistance when it is not.
- If the relay fails any test, it needs to be replaced — relays cannot be repaired.
Locate the relay and identify its pins
Most relays are cylindrical or rectangular modules with four, five, or eight pins sticking out of the bottom. The relay sits in a socket so you can pull it straight up and out. If you are not sure which component is the relay, check your equipment's manual or look for a part labeled "relay" on the circuit board or wiring diagram.
Once you have the relay in hand, look at the bottom. You will see pins numbered 1 through 8 (or fewer, depending on the type). The manual or a diagram printed on the relay itself will tell you which pins belong to the coil circuit and which belong to the contact circuit. If there is no label, search online for the relay's part number — most manufacturers publish pinout diagrams. Write down which pins you need to test before you start.
Test the coil for continuity and resistance
The coil is the electromagnet that responds to the signal. To test it, set your multimeter to the resistance setting (the symbol that looks like an omega: Ω). Touch one probe to one coil pin and the other probe to the other coil pin. A healthy coil usually reads between 50 and 200 ohms, though this varies by relay type — check the manual for the expected range.
If the reading is infinite (the display shows "1" or "OL" for overload), the coil is burned out and the relay must be replaced. If the reading is zero or very close to zero, the coil windings are shorted and the relay is also bad. If the reading falls within the expected range, the coil is probably good and you can move on to testing the contacts.
Test the contacts when the relay is not energized
The contacts are the metal switches that actually carry the high-power current. To test them, you need to check them in two states: when the relay is off (not energized) and when it is on (energized). Start with the relay off.
Set your multimeter to continuity mode (usually marked with a sound-wave symbol) or to the lowest resistance setting. Touch one probe to one contact pin and the other probe to the other contact pin. When the relay is off, the contacts should be open — the multimeter should show infinite resistance or no beep. If it shows zero or near-zero ohms, the contacts are stuck closed and the relay is bad.
Energize the relay and test the contacts again
Now you need to send power to the coil so the relay switches on. If the relay is still in its socket on a circuit board, you may be able to trigger it by pressing a button or turning on the equipment. If you have pulled the relay out, you will need to apply power to the coil pins yourself using a separate power supply or battery that matches the relay's voltage rating — usually 5V, 12V, or 24V. Check the manual to be sure.
While the coil is energized, touch your multimeter probes to the contact pins again. This time, the contacts should be closed — the multimeter should show zero or near-zero ohms, or a beep in continuity mode. If the reading is still infinite or very high, the contacts are stuck open and the relay is bad. If the reading drops to zero when you energize the coil and rises back to infinite when you de-energize it, the relay is working correctly.
What to do if the relay fails a test
Relays are sealed components and cannot be repaired or cleaned. If the coil reads infinite resistance, if the contacts are stuck in either position, or if the contacts do not change state when you energize the coil, the relay must be replaced. Buy an exact replacement — relays are inexpensive, usually between five and twenty dollars, and using the wrong type can damage the circuit or cause the equipment to malfunction.
When you install the new relay, make sure it sits fully in the socket and that all pins are making contact. If the new relay fails the same tests within a short time, the problem may not be the relay itself — something else in the circuit may be drawing too much current or sending the wrong signal to the coil. In that case, have the circuit board or wiring inspected by someone with experience in that type of equipment.
Common mistakes when testing relays
The most common mistake is testing the wrong pins. Always double-check the pinout diagram before you touch the multimeter to anything. Testing a contact pin as if it were a coil pin, or vice versa, will give you meaningless results and may lead you to replace a good relay.
Another mistake is forgetting to energize the coil when you test the contacts. The contacts will always read as open if the coil is not powered. If you are testing a relay that is still in the circuit, make sure the equipment is turned on or the button is being pressed while you take your reading. If you are testing a pulled relay, do not skip the step of applying power to the coil pins — it is the only way to know whether the contacts actually move.
Finally, do not assume a relay is bad just because one test reads high. A coil that reads 500 ohms might be perfectly normal for that relay type, even though it is higher than you expected. Always check the manual or the pinout diagram to see what the expected range is before you decide the relay has failed.
Frequently Asked Questions
Can I test a relay without removing it from the circuit?
Yes, if the relay is in a socket you can test it in place. However, you will get clearer results if you pull it out first, because the rest of the circuit will not interfere with your readings. If you test it in place, make sure the equipment is powered off before you touch the multimeter probes to the pins.
What voltage should I use to energize the coil when testing?
Use the voltage printed on the relay itself or listed in the manual. Common voltages are 5V, 12V, and 24V. Using the wrong voltage can damage the coil or give you a false reading. If you are not sure, do not guess — look it up first.
My relay reads 0 ohms on the coil test. Is it bad?
Yes. A coil that reads zero ohms means the windings are shorted together, and the relay will not work correctly. It needs to be replaced. A healthy coil always has some resistance — usually between 50 and 200 ohms, depending on the type.
The contacts read infinite resistance even when I energize the coil. What does that mean?
It means the contacts are stuck open and will not close even when the relay is powered. The relay is bad and must be replaced. This usually happens when the contacts corrode or wear out from repeated use.
How do I know which pins are the coil and which are the contacts?
Check the relay's part number and search for its pinout diagram online, or look in the equipment manual. The diagram will label each pin. If you cannot find the diagram, do not guess — testing the wrong pins will give you useless results and waste your time.