What 393.40A-B means in plain language
When an inspector cites you under 393.40A-B, it means your commercial motor vehicle was found to have braking equipment that doesn't meet the minimum performance standards required by federal regulation. That's not just a loose adjustment or a worn pad—it's a finding that the braking system as a whole falls short of what's needed to safely control and stop the vehicle under normal operating conditions.
The regulation covers both the adequacy of the brakes installed on the vehicle and whether those brakes can actually perform to the required standard when called upon. In other words, having brakes present isn't enough; they have to work to spec. Inspectors evaluate brake performance as part of a broader mechanical review, and if the system can't demonstrate it meets those thresholds, this code gets written up.
A CSA severity weight of 8 out of a possible maximum is assigned to this violation, which places it firmly in the serious end of the spectrum. Even a single citation carries meaningful weight in your Safety Measurement System (SMS) score, so understanding exactly what happened and what comes next matters.
What our enforcement data actually shows
Across our 13 million+ inspection records, 393.40A-B has been cited 3,236 times all-time, with 2,082 of those citations occurring in just the last 12 months. In the last 90 days alone, our database recorded 391 citations—a pace that signals active enforcement attention on this code.
The out-of-service picture is more nuanced than you might expect. Of all 3,236 all-time citations, only 122 resulted in a vehicle being placed out of service—an OOS rate of 3.8%. Compare that to the all-FMCSR average OOS rate of 31.4% across all codes in our database, and 393.40A-B sits dramatically below the norm. The code is OOS-eligible, so inspectors retain discretion, but in practice the vast majority of drivers—3,114 out of 3,236—kept moving after receiving this citation.
The monthly trend over the last year tells an important story. Citations spiked through the summer and fall months, peaking at 251 in August 2025 and 231 in October 2025, before dropping to 116 in December 2025. The year opened with 124 citations in January 2026 and climbed back to 185 by March 2026. That seasonal pattern suggests enforcement intensity fluctuates, but this code is consistently active—it ranks #423 out of 3,036 FMCSR codes by citation volume nationally.
Who gets cited most
California dominates the enforcement landscape for 393.40A-B. In the last 180 days, CA generated 321 citations—by far the highest volume of any state. Notably, 13 of those resulted in OOS placements, putting California's OOS rate at 4.0% for this code. Pennsylvania came in second with 54 citations at a 0.0% OOS rate, and Florida was close behind with 53 citations, also at 0.0%. The gap between California's OOS rate and those of Pennsylvania and Florida is meaningful—if you operate in California, inspectors there are more likely to pull your vehicle out of service on this violation than inspectors in most other states.
Looking at the vehicle makes most frequently cited, our data shows Dodge vehicles appear in 431 citations all-time—the most of any make—followed by Freightliner (FRHT) at 400 and Ford at 330. Kenworth (KW) and Peterbilt (PTRB) also appear in the top cited makes with 178 and 131 citations respectively. If you operate any of these platforms, the data suggests your make gets scrutinized on brake adequacy more than most.
On the carrier side, our data shows fleets such as SUPLICIUM TRANSPORT LLC (USDOT 4381255) with 15 citations and BETA AUTO TRANSPORT LLC (USDOT 4333977) with 12 citations have accumulated the most all-time appearances under this code across our records.
How severe is this compared to similar codes
Within the Vehicle Maintenance category, 393.40A-B is a relatively low-volume code compared to its peers, but its CSA weight makes it punch above its size. For comparison, 393.9(a) — Inoperable required lamps — has been cited 660,737 times in our database with a 15.4% OOS rate. That's two hundred times the citation volume of 393.40A-B. Meanwhile, 396.3(a)(1) — Inspection/repair/maintenance - general — carries 236,919 citations and a striking 45.3% OOS rate, meaning nearly half of drivers cited under that code get parked on the spot.
Another peer code worth noting is 393.47E — Slack adjuster defective — with 180,363 citations and a 0.0% OOS rate. That code shares both a mechanical relationship and real enforcement overlap with 393.40A-B, as we'll discuss below. The key takeaway: 393.40A-B's 3.8% OOS rate is lower than most of its category peers, but its severity weight of 8 means the SMS hit is real regardless of whether you got parked.
How to avoid it
The co-occurring violation data in our inspection records points directly at what inspectors are finding alongside 393.40A-B—and that pattern maps to a clear pre-trip checklist.
- Check slack adjusters every pre-trip. 393.47E (Slack adjuster defective) appeared in 49 of the same inspections as 393.40A-B in the last 90 days. Manually check each slack adjuster for travel and proper adjustment before you roll. An out-of-adjustment slack adjuster is one of the fastest paths to an inadequate brakes finding.
- Verify your periodic inspection documentation is on the truck. 396.17C-PI (No proof of periodic inspection) co-occurred in 58 shared inspections. If you can't hand the inspector a current inspection report, you're giving them a reason to dig deeper.
- Inspect relay and emergency valves. 393.43D-B (Brake - relay emergency valve) appeared alongside 393.40A-B in 38 shared inspections. During pre-trip, listen for air leaks around the valve housing and verify the system builds and holds pressure correctly.
- Don't ignore the brakes-out-of-service threshold. 396.3A1-BOS appeared in 104 shared inspections—the most common co-occurring code by far. That code triggers when 20% or more of your service brakes are defective. Walk your axles and check each brake chamber and lining condition before departure.
- Check your warning devices and lamps. 393.95F (missing stopped vehicle warning devices) and 393.9A-LLPL (inoperable required lamps) both co-occurred frequently. An inspector who writes up missing triangles or a dead clearance light is already looking hard at your mechanical condition.
- If you operate a Dodge, Ford, or Freightliner platform, be especially thorough. Our data shows these three makes account for over 1,100 of the 3,236 all-time citations under this code. Brake system maintenance intervals and component wear patterns on high-citation platforms deserve extra attention during pre-trip and scheduled PM.