§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
A horn citation shows up in Roadworthy HQ as part of the inspection it came from, not as a record of its own: the app does not store individual violation codes. Log the roadside inspection on the vehicle's Inspections tab: date, state, level, driver, the number of violations, and whether it was out-of-service. For an out-of-service inspection the 15-day §396.9(d)(3) repair-certification countdown stays on that tab until you enter the certification date. The rule's 15 days cover every violation on the report, not only out-of-service ones, so for a non-OOS inspection that date is yours to track. The repair itself goes in the vehicle's maintenance log: date, type, what was done, who did it, and cost. DVIRs are logged per vehicle with the defect described in your own words, and a defect marked safety-affecting that is logged without a repair certification sends a critical alert.