Horn Buying Guide
Last updated: 24 April 2026
The vehicle horn is a safety device that alerts other road users to your presence. A non-functioning horn is an MOT failure and a road safety hazard. Horn failures are usually electrical — the relay, fuse, wiring, or horn unit itself can be at fault. Replacement is straightforward and inexpensive.
Horn fault diagnosis
Horn problems are usually simple to diagnose. The circuit consists of a fuse, relay, horn button (on the steering wheel), wiring, and the horn unit itself.
- No sound at all when the horn button is pressed
- Weak, distorted, or intermittent horn sound
- Horn sounds continuously without pressing the button — relay or switch stuck
- Horn works sometimes but not always — intermittent wiring or corroded connector
- Check the fuse first — it is the most common and cheapest cause
Replacement tips
The horn is usually mounted behind the front bumper or grille. Access varies by vehicle.
- Check the fuse and relay before replacing the horn unit
- Test the horn by applying 12V directly to its terminals — if it sounds, the unit is good
- Replace corroded connectors and clean earth points
- Some vehicles have two horns (high and low tone) — replace as a pair for the correct sound
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Frequently Asked Questions
Is a broken horn an MOT failure?
Yes. The horn must be audible and operational to pass the MOT. It is classified as a failure, not an advisory.
How do I test if the horn unit is faulty?
Disconnect the horn connector and apply 12V directly from the battery. If it sounds, the unit is good and the fault is in the relay, switch, or wiring. If it does not sound, replace the horn.
Why does my horn sound weak?
A weak horn usually indicates internal corrosion or a worn diaphragm. Water ingress from road spray is the most common cause. Replace the horn unit.
Can I fit an aftermarket horn?
Yes, but ensure the replacement does not exceed noise limits. Air horns and multi-tone horns may not comply with regulations for standard road vehicles.
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