How To Tell If A Relay Fuse Is Blown?

Every vehicle has electrical components that enable efficient engine performance and a seamless driving experience. Some of these electronic and electrical components are regulated or controlled by a relay fuse. 

Whenever any electrical system ceases to function, there are chances that you have a blown relay fuse. Relay switches signal the components they’re assigned to work with so they will function properly.

For example, when you press the horn switch, the horn relay receives the input and signals the horn to blow. Common problems that could cause the relay to fail are exposure to water, a resistance problem, or computer failure. 

Here, RX mechanic teams have provided a concise guide on how to tell if a relay fuse is blown. But first, what does a bad relay look like? You’ll find out in the subsequent sections. 

How Do Relays Work In A Car?

how to test a relay

Read Also: What Does A Blown 30 Amp Fuse Look Like?

Relays come in various capacities, sizes, and corresponding uses. Although their capacities, sizes, and uses differ, they work the same way. One switch is used to power another.

Car relays are electrical switches controlled by electric flow like a computer module and another switch. The purpose of a car relay is to automate and switch on and off electrical contact.

In any case, relays have augmentative functions. They provide room for the driver to switch multiple switches (with different voltages) simultaneously.

12DC relay fuses at the best relays for full voltage components and applications. They can allow low electric flow to control high electric flow.

How To Tell If A Relay Fuse Is Blown 

There are several ways of checking a blown relay fuse. We’ll examine all of them in the following paragraphs. But first, let’s look at how to tell if a relay fuse is blown without a multimeter.

Visual check 

First off, locate the relay fuse box. This component is usually underneath the dashboard or under the hood. It is the black box next to the car battery. The relays are housed in the same box as the fuses on some vehicles. In other cars, the manufacturers place them in a separate compartment from the fuses. 

Once you locate the relay fuse box, have your assistant turn the ignition to the ON position, and you place your hand on the relay. You should hear and feel a clicking sound. The relay is long gone if it doesn’t make any clicking sound.

Return the ignition key to the OFF position for further examinations and pull off the relay. Connect a small wire on the two relay contact pins. Ensure you figure out the correct pins before connecting the wire.

The pins are usually called C (commons) and NO (normally open). On Bosch Automotive Relay, these pins have 30 and 87 inscriptions. After connecting the wires, re-plug the relay. Next, turn the ignition to the ON position, and see if the relay will click. If you hear a clicking sound, it’s a confirmation that the relay is bad. 

Swapping the relay 

 Another way to tell if you have a bad relay is to swap it with another good one. It works; the former relay in that sitting is bad. But if it doesn’t work, you have another issue to deal with. First, examine and clean the connectors. Then, try it again and see if it works. 

Using a multimeter 

A more simplified means of testing a relay is using a multimeter. First, get your multimeter and swipe it to ohm settings. Next, measure the resistance by touching multimeter probes on the relay electromagnet coil pins. The measurement should be around 50 – 120. Any result out of this range shows a malfunctioning electromagnet coil.

Using a car battery 

It’s also essential to learn how to test a relay with a 12v battery. This knowledge will be handy for a service technician and a DIY car owner. It requires a combination of a 12 volts battery and a multimeter. 

You can use the 12 volts battery to energize the electromagnet coil pins. You should hear a click from the electromagnet coil as it closes a switch.

Place the positive cable on one of the relay pins. Connect a test light to the ground and the other switch terminal. The test light should illuminate. Disconnect the positive cable, and the light will turn off.

What’s next?

Test the voltage on the relay. Disconnect the test light and set your multimeter to DC. Place the probes on the relay connectors and see if it matches the battery voltage. Test the resistance on the connectors.

Remove the positive jumper wire and return the voltmeter to ohm settings. Test the connectors for resistance. The measurement should be close to zero. So 01, 02 is okay.

Read Also: What Does A Blown 40 Amp Fuse Look Like?

do relays go bad

What Causes A Relay To Fail?

Now, you know that a relay switch on car cans fails. Therefore, you’ll no longer ask, ‘do relays go bad?’

Here are a few factors that’ll cause your relay to stop working.

Carbon build-up

If the mechanism inside the relay is stressed out or not making full contact because of higher electric flow, there will be excessive electric arcing. This will cause carbon build-up. Too much build-up may prevent the relay pins from connecting with the connectors on the sitting.

Heat, wear, and tear

Most electronic devices in your homes and offices suffer from heat, wear, and tear. The relay switch in your car is no exception. 

Excessive voltage

Electric current flows through the relays the moment you turn on your vehicle. Therefore, excessive voltage or bridge anywhere in the system can damage the corresponding relay.

Environmental and mechanical issues

Environmental issues such as corrosion and heat and mechanical damages like vibration can cause a relay to fail.

What happens if you ignore the problems?

Snapping in an old relay that fits in or ignoring this problem can lead to severe problems in the engine compartment. If you install a faulty relay or the vehicle relay is malfunctioning, it can burn electrical wires and possibly set up a fire in the engine compartment. Imagine this happens when cruising at 120km/h. You don’t want this to happen, even when idling the vehicle.

Read: What Does a Blown 50 Amp Fuse Look Like?

How To Replace Blown Relay Fuse

If you figure any of your relays are faulty, you may want to know how to fix a relay fuse. The only option I’d recommend is to replace it.

Luckily, replacing a blown relay fuse is replacing a 12 volts battery in a car. All you have to do is to locate the blown relay.

It could be somewhere in the engine compartment, usually close to the battery or underneath the dashboard, next to the driver’s door.

Open the relay fuse box. Look at the diagram on the cover to aid your search. The graph on the cover will project the directions of all the fuses and the relay.

Once you locate the blown relay, pull it out with a pair of pliers and snap in the new relay. That’s it. Cover the box, and you’re done.


Q: Is the relay a sensor?

No, a relay is not a sensing device. Instead, it is a switching device that consists of input connectors for multiple or single control signals and works as an operational terminal.

In other words, relays are switches that aim at opening and closing circuits electromechanically and electronically.

Q: What is the difference between a switch and a relay?

We described a relay as a switching device. However, it has several differences from a switch. Here’s a comparison between a relay and a switch.

Switch Relay
Switches are mechanically controlled Relays are electronically controlled
It controls electric flow by opening or closing circuits Relays control high electric flow on the power circuit by opening or closing the contacts
Switches are operated manually by pressing a button or by pulling a puller It can send an optical or electromagnet signal to activate the load circuit
Switches are used to open or close circuits. Relays are used to prevent systems from possible damage.
It works slower than relays because it needs a physical operation. It works faster than switches.
Switches make a direct connection or contact. It has a remote connection or contact.

Q: What is the difference between a fuse and a relay?

Relays and fuses perform similar roles. They also have similarities and differences. However, here’s the typical difference between them. A fuse is a device designed to interrupt a circuit once. In contrast, a relay connects or interrupts the circuit.

Q: How much does it cost to test a relay?

The cost of replacing a relay depends on several factors. First, you need to check if you want to replay it yourself or contact a service technician to install a new one.

The part will cost around $2 to $20, depending on the relay and your vehicle. If you choose to have a mechanic replace it, budget to spend $32 to $55. However, the price can be lower or higher than that.

Read: What Does A Blown 60 Amp Fuse Look Like?

Final Words

At this juncture, you have learned how to tell if a relay fuse is blown. The methods on how to check a blown relay fuse above are effective. You can use any of them and be sure of an efficient result.

However, I recommend the visual check method for all DIYers. This method is the most simplified guide and requires no tools. Although if you have the tools mentioned in this article, you can explore any of the methods.


Hi there, I am R. Hasan Tito, a mechanic, and owner of this website. My friend and I created this website to share our knowledge, expertise, and experience with our fellow mechanics' community and car users. I am a specialist and certified automotive mechanic (Both Heavy Commercial and Private Cars). I worked as a Mechanic and Mechanic Supervisor for over fifteen years at Global Rebound Automotive companies - Toyota, TATA, BMW, Nissan, TVs, and Others. Now, I enjoy my new role of leading a team of automotive experts (in their respective fields) and publish new content on a regular basis on my website and social media.

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