Violation code

393.17(b)

No/defective towaway lamps on rear unit

Vehicle Maintenance6Non-OOS· SMS Appendix A v3.20

393.17(b) is a Lighting violation under 49 CFR §393.17(b). It falls in the Vehicle Maintenance BASIC with a severity weight of 6/10. Nationally it was cited 4,811 times in the last five years, with an OOS rate of 91.3%.

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National roadside inspection stats

as of 10/20/2026

Inspections

3,746

that found this code

Violations

4,811

total written

OOS rate

91.3%

4,392 OOS

Source: FMCSA roadside inspection data (last 5 years). The OOS rate is the share of these violations that led the officer to mark the driver or vehicle out of service.

Violation group: Lighting
Regulation §: 393.17(b)
Severity weight: 6
Used in SMS: Yes
Status: Active
Updated: 10/20/2026

Last updated October 2026

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Notes

DSMS: Y

What 393.17(b) means — and how to avoid it

In plain English

In plain terms, 393.17(b) is cited for no/defective towaway lamps on rear unit. It falls under the FMCSA rules on parts and accessories needed for safe operation.

What the inspector checks

•Required lamps present, clean, and working — headlamps, tail, stop, turn, marker
•Reflective sheeting/reflectors intact
•Trailer cord and connections making contact

How to prevent it

✓Walk the lights on every pre-trip (have someone confirm brake/turn)
✓Carry spare bulbs/fuses and swap a dead lamp before rolling
✓Clean lenses and check the trailer plug for corrosion

How it affects your CSA

up to 18 pts (fresh)

Severity 6/10. A fresh citation counts at the 3× time weight — up to 18 points toward the Vehicle Maintenance BASIC — then tapers to 2× after 6 months and 1× after a year, dropping off the score at 24 months.

General guidance from the violation’s FMCSA classification — not legal advice, and not a substitute for the inspector’s findings or 49 CFR.

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Challenging 393.17(b) through FMCSA DataQs

If this violation was written incorrectly, here are the angles trucking-safety managers use to contest it. Playbook: Vehicle Maintenance (general).

Common ways to challenge this code

Pre-trip inspection showed compliant brakes
The driver's pre-trip DVIR documented compliant brake adjustment and operation. The cited defect arose en route or was misclassified at roadside.
Evidence that wins
- ✓Driver Vehicle Inspection Report (DVIR) from that date
- ✓Repair invoice or shop record
- ✓Post-citation re-inspection certificate (clean)
Momentary failure exception (§393.9)
The required lamp was inoperative only momentarily due to vibration, temperature, or similar transient cause — explicitly exempted under §393.9.
Evidence that wins
- ✓Photograph showing the lamp operable before / after the stop
- ✓Repair invoice if a fuse or bulb was replaced
Tread measurement disputed
The tread depth measurement was taken at a worn point that is not representative, contrary to NHTSA/FMCSA measurement standards.
Evidence that wins
- ✓Independent tread gauge reading from same tire (multiple positions)
- ✓Photograph of the tire with a reference scale
Documented maintenance schedule disproves the defect
Carrier's periodic-inspection records show the cited defect could not reasonably have existed at the time of inspection.
Evidence that wins
- ✓Periodic inspection certificate (§396.17)
- ✓Maintenance work order proximate to inspection date

Common pitfalls — avoid these

⚠DataQs cannot remove a violation simply because the defect was repaired after the stop

⚡ Draft a challenge for 393.17(b) →Read the DataQs guide first

More codes in Vehicle Maintenance

Same BASIC category — ranked by national citation volume

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Codes under the same FMCSA violation group

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Frequently asked questions about 393.17(b)

What does violation 393.17(b) mean?

393.17(b) is the FMCSA roadside-inspection violation code for "No/defective towaway lamps on rear unit", part of the Lighting group, cited under 49 CFR §393.17(b).

What BASIC does 393.17(b) affect?

393.17(b) falls under the Vehicle Maintenance BASIC in the FMCSA Safety Measurement System (SMS).

What is the CSA severity weight of 393.17(b)?

393.17(b) carries a severity weight of 6 out of 10. Higher weights indicate violations more closely tied to crash risk, and they count more toward a carrier's Vehicle Maintenance score.

Is 393.17(b) an out-of-service (OOS) violation?

No. 393.17(b) is not an out-of-service violation, so on its own it does not trigger an out-of-service order at the roadside.

How many CSA points does 393.17(b) add to a carrier's score?

The point contribution of 393.17(b) depends on its severity weight (6/10) multiplied by a time weight of 3, 2, or 1 based on how recent the inspection is. You can calculate the exact contribution with the CSA points calculator.

How can a driver prevent 393.17(b)?

Walk the lights on every pre-trip (have someone confirm brake/turn); Carry spare bulbs/fuses and swap a dead lamp before rolling; Clean lenses and check the trailer plug for corrosion.

Can 393.17(b) be challenged through DataQs?

Yes. If you believe 393.17(b) was recorded in error, you can file a Request for Data Review (RDR) through the FMCSA DataQs system with supporting evidence. A successful challenge can remove or correct the violation on your record.

Keep going

CSA points calculator How to challenge a violation Draft a DataQs letter for 393.17(b)Vehicle Maintenance BASIC Carrier CSA lookup CSA glossary

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