How do you check if a horn relay is bad?

In short, you determine a faulty horn relay by listening for a clear click when the horn button is pressed, testing the relay’s coil resistance and contact continuity, bench-testing the unit with a 12-volt supply, and, if needed, swapping it with a known-good relay to confirm the fault. This approach helps isolate the relay from the vehicle’s wiring and identify whether the relay is at fault or if another component in the horn circuit is the culprit.

What a horn relay does and why it can fail

The horn relay acts as a switch that draws current from the battery to the horn when you press the steering wheel horn button. When energized, the relay coil pulls the internal switch, connecting terminal 30 (power) to terminal 87 (the horn circuit). Failure can occur due to burned or shorted coils, corroded or loose connections, or worn contacts, leading to no horn, intermittent horn, or a constantly sounding horn in some miswired cases. Understanding these basics helps guide the diagnostic process.

Bench testing a horn relay

Use these bench steps to verify the relay’s electrical behavior outside the vehicle environment.

• Tools and parts: a multimeter (ohms/continuity), a 12V power source or car battery with jumper wires, a test load or the horn itself on a bench, and the horn relay.


• Identify terminals: for most 12V automotive relays, coil terminals are 85 and 86; switched terminals are 30 (input) and 87 (output). Some relays include 87a for normally closed contacts.


• Measure coil resistance: disconnect the relay from any circuit and measure between 85 and 86. Typical 12V coils read roughly 60–200 ohms depending on model; an open or very high resistance indicates a bad coil.


• Check coil continuity: ensure there is no short to the chassis or to other terminals when the coil is disconnected from power.


• Bench energize: apply 12V across the coil terminals (one lead to 85, the other to 86) and listen for a distinct click—the audible sign the coil is energizing and the switch is moving.


• Test the contacts: with the coil energized, connect a 12V source to terminal 30 and connect the load to terminal 87; the load should receive voltage when the coil is energized, indicating closed contact.


• Polarity note: most automotive relays are non-polarized, so coil polarity generally isn’t important; swapping 85/86 usually won’t affect operation, but confirm with the datasheet if available.


• Assess for sticking or arcing: if the relay clicks but the load doesn’t energize, the contact may be dirty, burned, or physically stuck; replacement is likely needed.

In bench tests, a relay that fails to click, shows abnormal coil resistance, or does not complete the circuit on the contact side is typically defective and should be replaced. If the bench test passes, the fault may lie elsewhere in the circuit.

In-vehicle testing and swap method

Diagnosing in the car accounts for actual wiring, fuses, and the horn switch. Follow these steps to confirm whether the relay is the culprit without guessing at the circuit.

• Check the horn fuse and wiring: locate the horn fuse in the fuse box and inspect it visually; test whether voltage is present at the relay input when the horn button is pressed.


• Inspect connectors: examine the relay socket and wiring harness for corrosion, bent pins, loose connections, or damaged insulation; clean or replace as needed.


• Test the horn switch circuit: with the ignition on, press the horn button and listen for a relay click. If there is no click, the issue may be the switch, wiring, or fuse rather than the relay.


• Measure voltages in the circuit: with a multimeter, check voltage at the relay coil terminals (85/86) and at the input (30) and output (87). Depending on the design, you should see 12V on one coil terminal when the button is pressed and a return path (ground or battery) on the other.


• Check the horn load: ensure the horn itself is functioning; temporarily substituting a known-good horn can help confirm the load isn’t the problem.


• Swap test: if you have access to a known-good relay from the same vehicle or a donor, swap it in. If the horn works with the spare relay, the original relay is likely at fault.

In-vehicle checks that reveal no click, no voltage at the horn feed, or persistent horn failure despite a good relay point to a switch, wiring harness, fuse, or horn component rather than the relay itself.

Symptoms that point toward a bad relay (and what to do next)

These signs commonly indicate relay or related circuit issues. Use them to prioritize your diagnostic steps and decide whether a replacement is warranted.

• No horn sound when pressing the horn button, or intermittent horn operation.


• Relays click inconsistently or not at all when the horn button is pressed.


• Fuses blow or circuits trip when the horn is activated, suggesting a possible short in the relay’s contacts or wiring.


• Corrosion, damaged pins, or loose connections at the relay socket.

If these symptoms persist after testing, focus on the switch, wiring, and horn itself. Replacing the relay is often a quick fix, but a full circuit check can prevent repeated failures.

Summary: A methodical approach—bench-testing the relay’s coil and contact health, validating control and power in the vehicle, and swapping with a known-good unit—provides a reliable way to determine if a horn relay is bad. This helps separate relay faults from horn or wiring issues and informs a practical replacement decision.

How does a horn relay work?

A horn relay works by using a low-current circuit to activate a switch that handles a high-current circuit for the horn. When you press the horn button, a small amount of current flows through a coil, creating an electromagnet. This electromagnet pulls a switch, which closes the high-current circuit, allowing full battery power to flow to the horn and sound it. This prevents the low-current horn button from being damaged by the high current the horn requires.
 
This video explains how a horn relay works in a car's electrical system: 24sCar FixYouTube · Jun 15, 2025
Detailed breakdown of the process

• Low-current control circuit: Pressing the horn button completes a low-current circuit that energizes the relay's coil. 
• Electromagnet activation: The current flowing through the coil creates a temporary magnetic field. 
• Switch closes: This magnetic field pulls a metal armature, which closes the high-current switch inside the relay. 
• High-current path: This action connects the high-current power from the battery (usually through a fuse) to the horn. 
• Horn sounds: The horn receives full battery power and operates, while the horn button and its delicate wiring are protected from the high-current load. 

You can watch this video to learn how to wire a horn relay in a car: 1mWiring RescueYouTube · Oct 9, 2021

How can I tell if my horn relay is bad?

The most effective way to diagnose this is to go directly to the horn and check for power at the connector. If there is power when the horn button is pressed, then the issue lies with the horn. If there is no power, then you should check the relay. Below are the images showing the locations of the horn and relay.

Can I bypass a horn relay?

Comments Section Yes, you'll be fine. I've done it before when a different relay has gone bad, swap the horn relay in and you're driving again until you get a new one.

Is there a way to tell if a relay is bad?

You can tell if a relay is bad by observing signs like malfunctioning components, flickering lights, or a no-start condition. To test it, use a multimeter to check the resistance of the coil (pins 85 and 86) to ensure it's within the expected range (typically 50-200 ohms). You can also apply a small voltage (like a 9V or 12V battery) to the coil to hear a "click," indicating it's activating. If it doesn't click, or if you don't get continuity between the switch contacts (pins 30 and 87) when the coil is energized, the relay is likely bad.
 
This video demonstrates how to test a relay using a multimeter: 1mMake It Easy MechanicYouTube · Dec 31, 2020
Signs of a bad relay

• Malfunctioning electrical components: Devices like your fuel pump, starter, or other components may not work correctly or at all. 
• Flickering lights: Headlights or dashboard lights may flicker, especially when the component controlled by the relay is active. 
• Vehicle won't start: The engine may crank but not start, or it may not crank at all. 
• Intermittent issues: The component may work sometimes but not others. 
• Clicking sound: A single click when the system is not working can indicate the relay is trying to activate but failing. 

How to test a relay
To perform a definitive test, you will need a multimeter and a 9V or 12V battery with jumper wires. 
Step 1: Test the coil resistance 

1. Remove the relay: from the vehicle. 
2. Set your multimeter: to the ohms (Ωcap omegaΩ) setting. 
3. Connect the multimeter leads: to the coil terminals, which are typically labeled 85 and 86. 
4. Check the reading. A good relay should show a resistance between 50 and 200 ohms. An open line (infinite resistance) or a very low reading can indicate a bad coil. 

Step 2: Test the switch contacts 

1. Connect the 9V or 12V battery: to the coil terminals (85 and 86) using jumper wires. 
2. Listen for a click, which indicates the relay is activating. 
3. Set your multimeter: to the ohms (Ωcap omegaΩ) setting again.
4. Check for continuity: between the switch terminals, which are typically 30 and 87. 
   • Before powering the coil, there should be no continuity between pins 30 and 87 (or very high resistance). 
   • While the coil is energized: (with the battery connected), there should be continuity between pins 30 and 87 (very low resistance). If there is no continuity when it should be there, the relay is bad. 

Step 3: Swap with a known good relay 

• If possible, swap the suspected relay with an identical one from a different circuit that is known to be working. 
• If the problem is resolved, the original relay was bad and needs to be replaced.