What fails a trailer on a DOT inspection?
Most trailers that fail a DOT inspection at Knox Trailers fail on three things: brake adjusters, lighting, and air leaks. All three we correct during the same visit, so a fail usually doesn't mean a second trip. The inspection itself covers the full federal list under 49 CFR 396.17, and we check brakes against the CVSA out-of-service criteria, which is the standard a roadside inspector uses.
Last updated November 12, 2026
The short version
Most trailers that fail a DOT inspection at Knox Trailers fail on three things: brake adjusters, lighting, and air leaks.
- The most common fails: brake adjusters, lighting, and air leaks.
- All three are corrected during the same visit.
- Brakes are checked against the CVSA out-of-service criteria.
- Structural defects are documented with photos and quoted before any work starts.
What do you check on the brakes?
Air system leak-down, slack adjuster travel, brake chamber stroke, lining wear, and the mechanical parts, all against the CVSA out-of-service criteria. A sticky slack adjuster is one of the fastest ways to an out-of-service violation.
What lighting problems fail a trailer?
Marker lights, brake lights, turn signals, the license plate lamp, reflectors, wiring harnesses, and the 7-pin connector. Bulb swaps and wiring repairs are handled in-house, usually while you wait.
What else gets flagged?
Kingpin wear, landing gear, frame cracks, crossmembers, the upper coupler, tread depth, mismatched tire sizes, lug nut torque, and suspension air bags and hangers. Structural defects get documented with photos and quoted so you can plan.
Common questions
What fails a trailer on a DOT inspection?
Most trailers that fail a DOT inspection at Knox Trailers fail on three things: brake adjusters, lighting, and air leaks. All three we correct during the same visit, so a fail usually doesn't mean a second trip.
What do you check on the brakes?
Air system leak-down, slack adjuster travel, brake chamber stroke, lining wear, and the mechanical parts, all against the CVSA out-of-service criteria. A sticky slack adjuster is one of the fastest ways to an out-of-service violation.
What lighting problems fail a trailer?
Marker lights, brake lights, turn signals, the license plate lamp, reflectors, wiring harnesses, and the 7-pin connector. Bulb swaps and wiring repairs are handled in-house, usually while you wait.
What else gets flagged?
Kingpin wear, landing gear, frame cracks, crossmembers, the upper coupler, tread depth, mismatched tire sizes, lug nut torque, and suspension air bags and hangers. Structural defects get documented with photos and quoted so you can plan.
Need the inspection this week?
Call the shop or request a quote and we will get the trailer on the schedule.