What 393.43D-B means in plain language
This regulation targets a specific component in your braking system: the relay or emergency valve. These valves are responsible for controlling air pressure delivery to your brakes and for triggering an automatic brake application if your air supply is suddenly lost — such as during a trailer breakaway. When either valve is defective or not functioning as designed, the entire braking system's reliability is in question.
Inspectors cite 393.43D-B when they find that a relay valve or emergency valve on a commercial motor vehicle is not working correctly. That can mean a valve that leaks, sticks, responds too slowly, or fails to hold pressure within spec. The defect doesn't have to be catastrophic to trigger the violation — a valve that's simply not performing to standard is enough.
For practical purposes: if an inspector puts their hands on your brake system and finds one of these valves misbehaving during a roadside inspection, this is the code that lands on your inspection report.
What our enforcement data actually shows
The numbers here are striking. Across our database of 13 million+ inspections, 393.43D-B has accumulated 14,687 all-time citations — ranking it #175 out of 3,036 FMCSR codes by citation volume. That puts it solidly in the top 6% of all codes for enforcement frequency.
But the out-of-service rate is what should get your attention immediately. Our inspection records show that 14,414 of those 14,687 all-time citations resulted in an out-of-service order — a 98.1% OOS rate. To put that in perspective, the all-FMCSR average OOS rate across our database is 31.4%. This code runs more than three times that average. When an inspector finds a defective relay or emergency valve, they almost universally pull the vehicle out of service.
Recent enforcement data reinforces that this isn't slowing down. Over the last 12 months, our records show 10,025 citations under this code. In just the last 90 days alone, 1,924 citations were recorded. The monthly trend data tells an even sharper story: citations climbed from 329 in April 2025 to a peak of 1,070 in July 2025, and have remained consistently above 600 per month since. Enforcement of this code is active and sustained.
Who gets cited most
Looking at the last 180 days of our inspection data, Tennessee leads all states with 326 citations — and every single one of those resulted in an OOS order, a 100.0% rate. Kansas follows with 237 citations and a 99.6% OOS rate. Pennsylvania comes in third with 218 citations and a 99.1% OOS rate.
Across the top ten states, OOS rates are remarkably consistent — most are at or near 100%. The one notable exception is California, where our data shows 212 citations but a 93.9% OOS rate. That 6+ percentage point gap compared to states like Tennessee, Florida, New York, Washington, Utah, Missouri, and Maryland is the only meaningful variation visible in the state-level data. Regardless of state, however, the dominant outcome when this code is written is a vehicle being placed out of service.
Our data shows fleets such as SAKARA LLC (USDOT 4429530) with 64 all-time citations and AUTO HAUL EXPRESS LLC (USDOT 4329325) with 58 all-time citations appearing at the top of the carrier citation counts in our database. These numbers reflect how frequently this specific defect surfaces across a fleet's inspection history.
How severe is this compared to similar codes
Within the Vehicle Maintenance category, 393.43D-B stands apart from most of its peers when it comes to OOS consequences. Consider a few comparisons from our data.
393.9(a), cited for inoperable required lamps, has accumulated 660,737 citations in our database — roughly 45 times the volume of 393.43D-B — but carries only a 15.4% OOS rate. Drivers get cited far more often for lamp violations, but they're sent down the road most of the time. That is not the case with 393.43D-B.
396.3(a)(1), covering general inspection, repair, and maintenance obligations, shows 236,919 citations and a 45.3% OOS rate in our records. That's a substantially higher OOS rate than the all-FMCSR average, but still less than half of what 393.43D-B produces.
396.17C-PI, the no-proof-of-periodic-inspection code, has 212,081 citations and a 0.0% OOS rate — it's a paperwork violation that never grounds a vehicle. The contrast with 393.43D-B's 98.1% rate illustrates exactly how differently inspectors treat a missing document versus a malfunctioning safety-critical valve.
How to avoid it
The co-occurring violation pattern in our data is a roadmap for what inspectors are finding alongside this code — and what you can address before you ever pull out of a scale or inspection station.
- Conduct a thorough air brake check on every pre-trip. With 393.48A-BIHE (inoperative/defective brakes) and 396.3A1-BOS (brakes out of service at 20%+ defective) both appearing in over 375 and 427 shared inspections respectively in our 90-day data, brake system integrity is clearly a consistent gap. Build time into your pre-trip to listen for air leaks at each valve location and verify pressure holds within spec during your static check.
- Inspect relay and emergency valves specifically — not just brake chambers. The valve itself is often overlooked during routine checks. Look for cracking, corrosion, or moisture intrusion at the valve body. If you're pulling a trailer, check the emergency valve connection between the tractor and trailer as part of your coupling procedure.
- Get your periodic inspection documentation current. Our data shows 396.17C-PI (no proof of periodic inspection) appeared in 612 shared inspections in the last 90 days alongside this code — the single most common co-occurring violation. A current, valid periodic inspection on file can reflect that brake components were recently evaluated by a qualified mechanic.
- Check your emergency equipment before every run. 393.95A1 (no fire extinguisher) and 393.95F (missing stopped-vehicle warning devices) each appeared in over 380 shared inspections in our 90-day data. These are quick pre-trip items that inspectors note alongside mechanical violations — don't let an equipment gap compound a brake citation.
- If your vehicle is a Ford, Dodge, or Chevrolet platform with a trailer connection, our all-time data shows these makes dominate 393.43D-B citation counts — Ford at 4,502, Dodge at 2,770, and Chevrolet at 1,313. On these vehicles, pay particular attention to the trailer brake valve connections and emergency air line fittings, which are common wear points on high-utilization units.