A relay is a switch that uses a small electrical signal to control a larger one — and when it fails, your car often won't start, or a major system stops working
A relay is an electromagnetic switch inside your car that lets a low-power circuit (like your ignition switch) control a high-power circuit (like your starter motor). When a relay fails, it either stays stuck open — blocking power from reaching what it controls — or stays stuck closed, draining power even when the car is off. You can test a relay yourself with a multimeter and a relay socket, or you can listen and feel for signs that one is failing.
The most common failure is an open relay: you turn the key and nothing happens, or a specific system (headlights, fuel pump, air conditioning) stops working while everything else runs fine. A stuck-closed relay is rarer but more dangerous — it drains your battery or causes a component to run constantly and overheat.
Key Takeaways
- A relay that fails open stops power from reaching a component, so that component won't work even though the rest of the car does.
- A relay that fails closed drains your battery or causes a component to run constantly, even when the ignition is off.
- You can test a relay with a multimeter by checking resistance across its terminals, or by swapping it with an identical relay from another circuit to see if the problem moves.
- A clicking sound from the relay box when you turn the key, or a relay that is hot to the touch, usually means the relay is failing.
- Relays are cheap and straightforward to replace once you locate the right one in your fuse and relay box.
Listen for a clicking sound when you turn the key
The most reliable sign that a relay is failing is a rapid clicking or chattering sound coming from the fuse and relay box (usually under the hood, sometimes under the dashboard). When you turn the ignition key or press the start button, a working relay makes a single, solid click. A relay that is failing often clicks repeatedly — sometimes dozens of times per second — because the internal contacts are sticking or bouncing.
This clicking usually means the relay is trying to close but cannot hold the connection. The component it controls (often the starter) may work intermittently, or not at all. If you hear this sound, the relay is almost certainly the problem. Locate the fuse and relay box, find the relay that corresponds to the system that is not working, and test or replace it.
Feel the relay for heat
A relay that is stuck closed will get hot because current is flowing through it constantly. If you can safely access the relay box while the car is running (or when ready after you turn it off), touch the relays with your finger or the back of your hand. A relay should be cool or only slightly warm. If one relay is noticeably hotter than the others, it is likely stuck closed and needs to be replaced.
Do not touch a relay when ready after the engine has been running — the whole box will be hot. Wait a minute or two, or use the back of your hand to test multiple relays at once so you can compare. A relay that is much hotter than its neighbors is the one to replace.
Test a relay with a multimeter
A multimeter can tell you whether a relay's internal contacts are working. Set the multimeter to the resistance setting (ohms, marked with the Ω symbol). Most relays have four or five terminals on the bottom. You are testing two things: the coil (the electromagnet that pulls the contacts closed) and the contacts themselves (the switch).
Consult your car's service manual or a relay diagram online to identify which terminals are which — this varies by relay type. Typically, you will measure resistance across the coil terminals (should read a few ohms to a few hundred ohms, depending on the relay) and across the contact terminals (should read nearly zero ohms when the relay is energized, or very high when it is not). If the coil reads open (infinite resistance) or the contacts are stuck open or closed, the relay is bad.
Testing a relay this way requires you to remove it from the car first, or to use a relay socket tester that lets you plug the relay in and measure without removing it. If you do not have a multimeter or are not comfortable using one, the swap test (below) is simpler.
Swap the relay with an identical one from another circuit
The fastest way to confirm a bad relay is to swap it with a relay that you know is working. Find an identical relay in your fuse and relay box — it will have the same part number printed on top. Relays that control different systems often have the same internal design, so a relay from your headlight circuit might be identical to one from your fuel pump circuit.
Remove the suspected bad relay by pulling it straight out of its socket. Note which socket it came from. Insert the identical relay from another circuit into that same socket. Start the car and see if the problem goes away. If it does, the relay you removed is bad. If the problem persists, the relay is not the issue — the problem is likely in the wiring, the component itself, or the switch that controls the relay.
Once you have confirmed the relay is bad, replace it with a new one (not the one you borrowed). Relays are inexpensive — usually between five and twenty dollars — and are sold at any auto parts store. Bring the old relay with you so the parts associate can match the part number.
Identify which relay controls the system that is not working
Your fuse and relay box has a diagram on the inside of the cover or in your owner's manual. This diagram shows which relay controls which system — starter, fuel pump, headlights, air conditioning, and so on. Locate the system that is not working, find the corresponding relay in the diagram, and note its position in the box.
If you cannot find the diagram, search online for "[your car year, make, model] fuse relay box diagram." Most manufacturers post these diagrams, and many auto parts websites have them too. Once you know which relay you are testing, you can remove it and perform the swap test or use a multimeter to check it.
Know the difference between a relay that fails open and one that fails closed
A relay that fails open breaks the circuit, so the component it controls stops working. Your car will not start, or your headlights will not turn on, or your fuel pump will not run. Everything else works normally. This is the most common relay failure.
A relay that fails closed keeps the circuit running all the time, even when it should be off. This drains your battery, causes a component to overheat, or makes a system run constantly. For example, a fuel pump relay stuck closed will run the fuel pump even when the ignition is off, draining the battery in a few hours. A starter relay stuck closed can cause the starter to spin even after you release the key. These failures are less common but more urgent to fix.
Both types of failure require relay replacement. The difference matters only for diagnosis: if a system does not work at all, suspect an open relay; if a system runs when it should not, suspect a closed relay.
Frequently Asked Questions
Can a relay fail partially, so a system works sometimes but not always?
Yes. A relay with corroded or worn contacts may work when the engine is cold but fail when it is hot, or work for a few seconds then stop. This is usually a relay that is failing open. The swap test will confirm it — if the problem disappears when you swap in a known-good relay, the original relay is bad even if it works intermittently.
What does it mean if I hear clicking but the component still works?
Clicking while the component works usually means the relay is on the edge of failure. The contacts are bouncing or sticking slightly, but they are still making a connection. Replace the relay soon — it will fail completely within days or weeks. Do not wait for it to stop working entirely.
Is it safe to drive with a bad relay?
It depends on which relay is bad. A bad starter relay means you cannot start the car at all. A bad fuel pump relay means the engine will stall. A bad headlight relay means you have no headlights. A bad relay that is stuck closed may drain your battery or cause a fire if the component overheats. Do not drive the car until the relay is replaced.
How long does it take to replace a relay?
Replacing a relay takes five to ten minutes. You remove the old relay from its socket, insert the new one, and test it. No tools are required. If you cannot locate the relay or are unsure which one is bad, diagnosis may take longer, but the replacement itself is quick.
Can I test a relay while it is still in the car?
Testing with a multimeter while the relay is installed is difficult because you have to reach the terminals and avoid touching other components. The swap test works better — pull the suspected relay, insert a known-good one, and see if the problem moves. If you want to use a multimeter, remove the relay first.