What a relay does and why it fails
A relay is an electromagnetic switch that uses a small electrical signal to control a larger one. In your car, relays handle high-current jobs — starting the engine, running the fuel pump, powering the cooling fan, controlling headlights — without forcing the ignition switch or computer to handle that load directly. When a relay fails, the circuit it controls either stops working or works erratically.
Relays fail because the internal contacts wear out, the coil burns, or moisture corrodes the terminals. A relay that clicks when you turn the key but the starter doesn't engage, or one that lets the fuel pump run constantly, is usually dead. Unlike a fuse, which straightforward breaks, a relay can fail in ways that are harder to spot — it might work sometimes and not others, or it might stick in the on position.
Key Takeaways
- A relay that has failed will often produce a clicking sound from the relay box when you try to use the circuit it controls, or the circuit will not work at all.
- Most relays are swappable — you can remove a relay of the same type from a different circuit, install it in the suspected bad relay's socket, and see if the problem moves with it.
- The relay box location and the correct relay type are shown in your owner's manual or on a diagram inside the relay box cover itself.
- A multimeter set to resistance mode can test whether a relay coil is open (burned out) or shorted, though a swap test is faster and more reliable for most owners.
- If swapping relays solves the problem, the original relay is bad and needs replacement; new relays cost between $10 and $30 depending on the type.
Locating your relay boxes and identifying the right relay
Most cars have two relay boxes: one under the hood (usually near the battery or fender) and one inside the cabin (often under the dashboard on the driver's side, or behind a kick panel). Some vehicles have relays scattered across multiple locations. Open your owner's manual and search for "relay box" or "fuse and relay box" — the manual will show you the exact location and include a diagram of which relay controls which circuit.
The relay box cover itself usually has a printed diagram showing each relay's position and function. If the cover is missing or illegible, take a photo of the box and search online for "[your year, make, model] relay box diagram" — automotive forums and parts sites have these diagrams for nearly every vehicle. Write down the relay number and its function (for example, "Relay 5: Fuel Pump") before you remove anything.
Relays come in standard sizes — most common are 4-pin, 5-pin, and 8-pin types. The number of pins determines which socket it fits into. You cannot put a 5-pin relay into a 4-pin socket, so matching the pin count is essential. The relay itself is usually labeled with its type and amperage rating.
The swap test: the fastest way to check a relay
The swap test is the most practical way to check a relay without tools. Find another relay in the same box that is the same type (same number of pins) and controls a circuit you do not need right now — for example, if you suspect the fuel pump relay, you might swap it with the horn relay. Write down which relay you are moving so you can swap them back.
Turn off the engine and remove both relays by pulling them straight out of their sockets. Install the known-good relay into the suspected bad relay's socket. Turn on the ignition and test the circuit — try to start the engine, turn on the headlights, or set up whatever the relay controls. If the problem moves to the relay you removed (now the horn does not work, or the fan does not run), the original relay is bad.
If the problem does not change — the fuel pump still does not run, for example — then the relay itself is probably not the cause. The issue may be a wiring break, a bad connector, a blown fuse, or a failed component downstream. Swap the relays back to their original positions and move on to checking the fuse and wiring for that circuit.
Testing a relay with a multimeter
A multimeter can tell you whether a relay coil is open (broken) or shorted (internally damaged). Set the meter to the resistance (ohms) setting. Remove the relay from the box and hold it so you can see the pins on the bottom. The coil pins are usually pins 1 and 3 (or 85 and 86 on 5-pin relays) — check your relay's label or the box diagram to confirm which two pins form the coil.
Touch one meter probe to one coil pin and the other probe to the second coil pin. A healthy relay coil will show a resistance reading between 50 and 200 ohms, depending on the relay type. If the meter shows infinity (no continuity), the coil is open and the relay is dead. If it shows zero or very low resistance, the coil is shorted and the relay is also bad. If the reading is within the normal range, the coil is intact — but this does not may provide the relay works, because the contacts inside can still be worn or stuck.
The contact pins (usually 30, 87, and 87a on a 5-pin relay) are harder to test without a power supply, so the swap test is more reliable for checking whether the contacts actually close and open. Use the multimeter coil test to rule out a burned-out coil, then use the swap test to confirm the relay actually switches.
When a relay clicks but the circuit does not work
A relay that clicks when you turn the key but the controlled circuit (starter, fuel pump, fan) does not engage usually has burned contacts. The coil is strong enough to pull the switch closed, so you hear the click, but the contacts are pitted or corroded and cannot carry current. This relay needs replacement — it will not recover.
Before you replace it, check the fuse for that circuit. If the fuse is blown, the relay may be clicking because it is trying to close a circuit that has no path. Replace the fuse first and test again. If the fuse blows again when ready, you have a short somewhere in the wiring, not a bad relay. If the fuse holds and the relay still clicks without the circuit working, the relay contacts are bad.
When a relay stays on or causes constant drain
A relay that is stuck in the on position will cause the circuit it controls to run constantly — the fuel pump runs even when the engine is off, the cooling fan runs all the time, or the headlights stay on. This is a different failure mode from a dead relay, but the solution is the same: replace it.
To confirm the relay is stuck, remove it from the box while the engine is off. If the circuit stops (the fuel pump shuts off, the fan stops), the relay was holding it on. Reinstall the relay and listen for a continuous hum or buzz from the relay box — a stuck relay often makes noise. Swap it with a known-good relay of the same type to confirm. If the problem stops after the swap, the original relay is stuck closed and must be replaced.
Replacing a failed relay
Once you have confirmed the relay is bad, buy a replacement of the exact same type. Take the old relay to an auto parts store and match it by part number, or order online using your vehicle's year, make, model, and the relay's function. New relays typically cost between $10 and $30. Some specialty relays (high-amperage, time-delay, or module-integrated types) can cost more, but standard relays are inexpensive.
Turn off the engine, remove the bad relay by pulling it straight out, and insert the new one into the same socket. Make sure it seats fully — you should hear or feel a click. Turn on the ignition and test the circuit. The problem should be gone. If it is not, the issue is not the relay, and you should have the wiring and fuse checked by a mechanic.
Frequently Asked Questions
Can I drive the car if a relay is bad?
It depends on which relay failed. If it is the fuel pump relay, the engine will not start or will stall. If it is the cooling fan relay, the engine may overheat. If it is a headlight relay, you lose headlights. Some relays control non-essential circuits like the horn or seat heaters, so the car is drivable but that feature does not work. Do not drive if the relay controls the starter, fuel pump, or cooling fan.
Why do relays fail so often?
Relays do not fail often — most last the life of the car. When they do fail, it is usually because of heat cycling (the coil expands and contracts), moisture inside the relay box, or a power surge from a bad alternator or battery. Relays in high-heat areas (near the engine) or in cars exposed to salt and humidity fail sooner than others.
Is it safe to swap relays to test them?
Yes, as long as you swap relays of the same type and pin count. Swapping a fuel pump relay with a horn relay temporarily will not damage either relay or the car. Just remember to swap them back, or the horn will not work and the fuel pump will not run. Write down which relay you moved before you start.
What if the relay tests good but the circuit still does not work?
The relay is probably not the problem. Check the fuse for that circuit first — a blown fuse will prevent the relay from doing anything useful. Then check the wiring and connectors for corrosion or loose terminals. If the fuse is good and the wiring looks clean, the issue may be a failed component (fuel pump, motor, solenoid) or a broken wire inside the harness. A mechanic with a wiring diagram can trace the circuit.