§393.55(a) requires every air-braked truck tractor manufactured on or after March 1, 1997, and every other air-braked CMV manufactured on or after March 1, 1998, to have an antilock brake system that meets FMVSS 121. Hydraulically-braked CMVs are also covered for vehicles manufactured on or after March 1, 1999. The dashboard ABS malfunction indicator lamp signals the system status — if lit while moving, ABS is not functioning correctly.
Severity
Severity weight 1 — low. The 20% brake rule that places vehicles OOS does not apply to ABS findings; an ABS lamp lit alone is not OOS. The risk is pattern findings on the same vehicle, indicating maintenance neglect.
What triggers the finding
- ABS warning lamp illuminated while the vehicle is moving (most common)
- Disconnected wheel-speed sensor at any wheel position
- Disconnected modulator or ECU
- Tractor with ABS lamp on the dash; trailer with ABS lamp on the side (FMVSS 121 trailer-ABS requirement, March 1, 1998 vehicles forward)
How to prevent it
- DVIR includes ABS-warning-lamp check at pre-trip — driver verifies lamp self-test at startup.
- Wheel-speed sensors take dirt and corrosion damage; clean and re-seat at every brake job.
- Trailer ABS is often overlooked — verify the trailer warning lamp at coupling.
How Roadworthy HQ helps
An ABS 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. The §396.17 annual inspection report uploads to the vehicle, with reminders 60, 30, and 7 days before the next one is due.