What 393.43 means in plain language
FMCSR 393.43 targets a specific but critical component in your braking system: the relay valve and the emergency valve. A relay valve is responsible for speeding up brake application and release on air brake systems — it takes a signal from the treadle valve and rapidly applies air pressure to the brake chambers without that air having to travel the full distance from the cab. When that valve is defective or not working as designed, your brakes respond slower or not at all.
The emergency valve serves a different but equally vital role. It's the system that automatically applies the brakes on a trailer if it becomes separated from the tractor or if air pressure drops below a safe threshold. A malfunctioning emergency valve means that built-in failsafe is gone. You could have a trailer with no effective braking protection in a breakaway situation.
When an inspector finds either of these valves defective — through a physical inspection, an air loss test, or by observing brake response — a 393.43 citation is written. Because this is a fundamental braking system failure, it is out-of-service eligible, and as the data below shows, inspectors exercise that authority at an extremely high rate.
What our enforcement data actually shows
Across our 13 million+ inspection records, 393.43 has generated 23,816 all-time citations, placing it at #114 out of 3,036 FMCSR codes by citation volume. That puts it well inside the top 4% of all cited codes — this is not an obscure or rarely enforced regulation.
The out-of-service numbers are striking. Of those 23,816 citations, 19,467 resulted in the vehicle being placed out of service — an OOS rate of 81.7%. To put that in context, the all-FMCSR average OOS rate across every code in our database is 31.4%. The 393.43 rate is more than 2.5 times that average. When an inspector writes this violation, placing the vehicle out of service is far and away the most likely outcome.
Enforcement is not slowing down. Our inspection records show 1,798 citations in the last 12 months and 356 in the last 90 days alone. Looking at monthly data, the numbers have been consistently high: 193 citations in May 2025, 180 in July 2025, and 165 in February 2026. Every month in the past year has produced triple-digit citation counts except April 2025 (69 citations) and April 2026, which was still in progress at the time of this data snapshot (10 citations, all resulting in OOS).
Who gets cited most
The three states generating the highest 393.43 citation volume in the last 180 days are Texas (288 citations), North Carolina (258 citations), and Illinois (147 citations). If you run lanes through any of these states, pay attention to the OOS rate differences — they are significant.
In Texas, our data shows a 70.1% OOS rate on 393.43 citations, meaning roughly 3 in 10 drivers cited there stayed on the road. In North Carolina and Illinois, the rate is essentially 100% — 258 out of 258 OOS in NC, and 146 out of 147 OOS in IL. New Mexico shows the same pattern: 61 citations, 61 placed out of service. If you're inspected in these states and an inspector finds a relay or emergency valve problem, you are not driving away.
Our data shows fleets such as MARYLAND TREE EXPERTS (USDOT 3304931) with 38 all-time citations and BENNETT TRUCK TRANSPORT LLC (USDOT 600382) with 26 citations appearing at the top of the carrier list. These numbers reflect that certain fleet types — including equipment haulers and specialty carriers — accumulate exposure across many vehicles and inspection events.
By vehicle make, FORD leads with 3,176 all-time citations under this code, followed by DODG (1,270) and TRLR (1,134). The presence of FORD and DODG at the top signals that medium-duty commercial vehicles — not just Class 8 semi trucks — are frequently cited under 393.43. If you're operating a Ford or Dodge-based CMV with air brakes, this code applies directly to your equipment.
How severe is this compared to similar codes
Within the Vehicle Maintenance category, 393.43 occupies a uniquely dangerous enforcement position when you look at OOS rate rather than raw citation volume.
Consider 393.9(a) — Inoperable Required Lamps, the most-cited code in the category with 660,737 citations. Its OOS rate is just 15.4%. Or look at 393.11 — Lighting Devices/Reflectors with 179,734 citations and a 1.8% OOS rate. These are high-volume codes, but inspectors rarely park a truck over them.
Contrast that with 396.3(a)(1) — Inspection/Repair/Maintenance (General), which carries a 45.3% OOS rate across 236,919 citations — itself well above the FMCSR average, but still nearly 36 percentage points below 393.43's 81.7% rate. The relay and emergency valve violation is in a different enforcement tier. Inspectors treat it as a critical brake system failure, not a paperwork or lighting issue.
How to avoid it
The co-occurring violation data from our inspection records makes the prevention picture clear. In the last 90 days, 393.43 appeared alongside 396.17C (No proof of periodic inspection) in 106 shared inspections, and alongside 393.48A (Inoperative/Defective Brakes) in 40 shared inspections. This is a vehicle that isn't being inspected or maintained on schedule, and when the inspector digs in, they find multiple brake-related failures.
Here's what you can do before every trip:
- Test your air brakes fully during pre-trip. Apply the brakes and check for abnormal lag in pressure build-up or release. A sluggish relay valve often shows up here before it fails completely.
- Listen for air leaks around the valve body. A relay valve leaking air at rest or during brake application is a defect — don't assume it's minor.
- Confirm your emergency/spring brake system charges and releases correctly. Hook up a trailer, charge the system, and verify the trailer brakes release fully and apply correctly when air is dumped.
- Document your periodic inspection and carry proof. The 106 shared inspections with 396.17C tell you that drivers getting hit for 393.43 often also have no proof of periodic inspection. Your last annual inspection should be in the truck.
- Pay extra attention on Ford and Dodge-platform CMVs. Our database shows these makes lead all citation counts under this code. If you operate one with air brakes, add valve condition to your standard pre-trip checklist by name.
- Don't combine brake problems with other deferred maintenance. Co-occurring codes like 393.9 (110 shared inspections) and 393.95A (95 shared inspections) show that vehicles cited for 393.43 frequently have lamp and emergency equipment failures too. A truck in that condition draws a thorough inspection — and a thorough inspection on a vehicle with a bad relay valve almost always ends in OOS.