§393.81 requires every CMV to be equipped with a horn that meets the requirements of SAE J377 (Performance of Vehicle Traffic Horns). In practice, that means a single, audible horn — the inspector tests it from outside the vehicle.
Severity weight
Severity weight 1 in the Vehicle Maintenance BASIC. The finding is common because the test is trivial — an inspector who reaches the cab will press the horn, and a non-functioning horn is a guaranteed cite.
How it gets cited
Three patterns drive most §393.81 findings: (1) a corroded ground connection at the horn relay or horn itself, (2) a blown horn fuse from a stuck horn ring, (3) a damaged steering wheel after a column repair that left the horn circuit disconnected. None of these signal poor maintenance; all of them produce the same cite.
How to prevent it
- Test the horn during every pre-trip — it takes one second.
- If the horn weakens or sounds different, replace before the next trip; failing horns rarely recover.
- After any steering-column or airbag work, verify the horn ring before the vehicle returns to service.
How Roadworthy HQ helps
DVIR pre-trip checklists in Roadworthy HQ include the horn test, and any defect entered on the DVIR follows the §396.11(c) repair-or-no-defect workflow so a §393.81 finding cannot be "fixed by ignoring it" between inspections.