The record for code 393.9A-LIL reads "Lighting - Identification lamp(s) inoperative." It sits under the Vehicle Maintenance BASIC in the violation group "Clearance Identification Lamps/Other." Before I get into scoring, it helps to know what the lamps actually do, because the code is easier to prevent once you understand the job.
Identification lamps are the cluster of small marker lights mounted across the top of a wide vehicle, front and rear. They show the vehicle's overall width and tell other drivers it is a large unit. Trailers carry their own set, wired through the same connector as the rest of the lighting. An inspector checks them by having the lights switched on and looking at each one. A single dead lamp is enough to write the code. You can read the full record on the violation page.
What the data says
As of October 20, 2026, our dataset attaches 173,681 violation records and 164,637 inspections to this code. It is used in SMS calculations and carries a severity weight of 2. Only 2 out-of-service entries appear against those 173,681 records, and the OOS flag is off, so the out-of-service rate rounds to zero.
Two things follow. This is a common finding, the kind of thing that shows up on a light bulb or a tired connector across thousands of inspections. And it almost never takes a truck off the road by itself. The cost comes from the scoring, not from the roadside.
How the points work
The calculator constants in our data use time tiers. A violation carries a time weight of 3 in its first six months, 2 from six to twelve months, 1 from twelve to 24 months, and 0 after 24 months. Multiply that by the severity weight of 2 and the arithmetic is simple: 6 points in the first six months, 4 in months six through twelve, 2 in months twelve through 24, then nothing.
The calculator also caps the severity contribution of one inspection to one BASIC at 30, so a single bad inspection cannot dominate the category. This code is not flagged out-of-service, so the OOS bonus does not apply.
The practical lesson is narrow. A dead identification lamp is a cheap repair, but it counts at full weight for the first six months regardless of whether the cause was a bulb or a corroded pin.
Where drivers lose it
Trailers. The usual failure is on a unit you did not pull in with. The lamps run through the trailer connector, and a corroded pin or a poor ground can take out the whole cluster while your tractor lights work fine. Many drivers assume identification lamps belong to the tractor. Check trailer lamps at every hookup, not just the first one.
Wrong replacements. A lamp swapped for a part that does not match the original socket or color can fail early or look wrong on inspection.
Daylight pre-trips. People walk the unit in daylight with the lights off and never see a dead lamp. A lamp that is out only shows up when it is switched on, so the check needs to include the switches.
What to do
- Turn on every marker and identification lamp during pre-trip. Do it at night if you can.
- If one lamp is out, check its neighbors. A single dead lamp usually points to the bulb or its socket. Several dead lamps on the same run point to wiring or a shared fuse.
- Inspect the connector and ground on any trailer you hook up.
- Repair it and write it up. A repair record helps when an inspector asks what you did.
Where to go next
The code is common and low-weight, which is exactly why it gets ignored. If you want to see how a mix of violations ages through the time tiers, the calculator shows how weights shift month to month. The regulation text itself lives in 49 CFR Part 393, and you should read the current version there rather than rely on a summary, including this one.
Key takeaways
- 393.9A-LIL is a Vehicle Maintenance item with a severity weight of 2 and no OOS flag in our data.
- In the calculator's time tiers, it carries 6 points in months 0–6, 4 in months 6–12, 2 in months 12–24, and 0 after that.
- Trailer connectors are the most common place the fault hides.
- Test every marker and identification lamp with the lights switched on, on every unit you drive.