What 393.9T means in plain language
FMCSR 393.9T addresses a straightforward but commonly overlooked maintenance failure: one or more of your vehicle's required tail lamps is not functioning. Tail lamps are the red lights mounted at the rear of your truck or trailer that signal your presence to other drivers, especially in low-visibility conditions like night driving, rain, or fog.
The regulation doesn't require much interpretation. Your tail lamps must work — period. A burned-out bulb, a broken socket, a corroded connector, or a wiring fault that kills the lamp all put you in violation. It doesn't matter whether the failure happened two miles ago or two months ago; if an inspector finds it dark at the rear of your unit, you're getting written up.
This is classified under Vehicle Maintenance, meaning it's treated as an equipment defect your pre-trip inspection is supposed to catch before you ever leave the yard.
What our enforcement data actually shows
Across our inspection records, 393.9T has accumulated 14,785 all-time citations, placing it at #174 out of 3,036 FMCSR codes by citation volume — firmly in the top 6% of all codes enforced. That's not a niche or rarely-enforced rule. In the last 12 months alone, our database recorded 8,814 citations, and 1,980 of those came in just the last 90 days. Enforcement is not slowing down.
The out-of-service picture is more forgiving than many codes. Our data shows an 8.9% OOS rate all-time — meaning 1,312 of the 14,785 citations resulted in the driver being placed out of service, while 13,473 did not. For context, the all-FMCSR average OOS rate across all codes is 31.4%. At 8.9%, 393.9T sits well below that average, which tells you inspectors generally view a single inoperable tail lamp as a citable defect rather than an immediate roadside shutdown.
That said, 8.9% is not zero. Roughly 1 in 11 drivers cited for this violation still got parked. Looking at the monthly trend in our records, October 2025 saw 86 OOS placements out of 782 citations, and December 2025 produced 85 OOS placements out of 716 citations — both well above the baseline rate. Winter months appear to carry elevated OOS risk for this violation, possibly because lighting defects become more consequential during shorter days and worse weather conditions.
Who gets cited most
Our inspection records for the last 180 days show Texas leading all states by a wide margin, with 3,156 citations for 393.9T. Iowa comes in second at 462 citations, and North Carolina third at 149 citations.
The OOS rate variation across those three states is significant and worth paying attention to. Texas enforces this violation at a 7.2% OOS rate — below the code's all-time average. Iowa runs slightly higher at 10.2%. But North Carolina stands out sharply at a 38.3% OOS rate on just 149 citations — more than five times the Texas rate. If you're running through North Carolina, the data strongly suggests inspectors there are far more likely to park you for a tail lamp violation than in Texas or Iowa. Illinois (18.4%) and New Mexico (15.4%) also run meaningfully above the Texas rate.
Among carriers in our all-time data, RS Transfer SA de CV (USDOT 1156825) appears with 31 citations, and Servicio Internacional de Enlace Terrestre SA de CV (USDOT 818175) appears with 30 citations. Several cross-border Mexican carriers make up a significant portion of the top-cited fleets, which is consistent with the heavy inspection activity at southern border crossing points — particularly in Texas, the dominant state in this code's enforcement footprint.
How severe is this compared to similar codes
Within the Vehicle Maintenance category, 393.9T is a relatively minor violation by most measures, but the broader lighting code family it belongs to is enormous in scale.
The parent code 393.9(a) — covering inoperable required lamps generally — has 660,737 all-time citations in our database with a 15.4% OOS rate. That's nearly 45 times the citation volume of 393.9T and a noticeably higher OOS rate, reflecting the fact that the general lamp code captures a wider range of more severe lighting failures.
Another peer code worth comparing is 393.11, which covers lighting devices and reflectors broadly and shows 179,734 citations with a 1.8% OOS rate — lower severity in terms of OOS risk but enormous volume. Meanwhile, 396.3(a)(1), the general inspection, repair, and maintenance code, carries 236,919 citations and a 45.3% OOS rate, which illustrates how quickly a pattern of deferred maintenance escalates beyond lighting violations alone.
The takeaway: 393.9T by itself is unlikely to shut you down in most states, but it is a high-volume code that feeds directly into your carrier's BASIC scores, and it frequently travels with other violations that collectively create a much more serious inspection outcome.
How to avoid it
Our co-occurrence data from the last 90 days makes the prevention picture very clear. In 1,980 recent inspections involving 393.9T, inspectors found additional violations at high rates — and the pattern tells you exactly where to focus your pre-trip.
- Walk the rear of the trailer every single pre-trip. Have someone stand at the cab and cycle the headlights and running lights while you confirm all tail lamps illuminate. Don't assume last night's working lamp is still working this morning.
- Check all rear lighting together. In our data, 393.9TS (inoperative turn signal) co-occurred in 669 of the same inspections, and 393.9H (inoperable headlamps) appeared in 195. A blown fuse or a bad ground often kills multiple circuits at once — if one is out, check them all.
- Inspect the trailer connection and glad hand area. Freightliner (FRHT) units account for 4,202 all-time citations, Kenworth (KW) for 2,357, and Peterbilt (PTRB) for 1,909 — the three most-cited makes in our database. On these platforms, corrosion at the seven-pin connector is a common cause of intermittent or dead tail lamp circuits. Clean and inspect the connector at every trailer swap.
- Don't skip emergency equipment. 393.95A (fire extinguisher missing or defective) appeared in 254 of the same inspections in our data. An inspector who finds your tail lamp out is walking the entire vehicle — make sure you're not handing them additional write-ups.
- Carry spare bulbs and fuses. This is the single cheapest and most effective prevention tool available. A $4 bulb found in your cab at roadside ends the conversation before it becomes a citation or a CSA point.
- Review your periodic inspection paperwork. 396.17C (no proof of periodic inspection) co-occurred in 304 of the same recent inspections. If your lamp was out and you can't show a current inspection record, you've turned one violation into two.