What 393.42 means in plain language
When you get cited for FMCSR 393.42, it means an inspector found that your commercial motor vehicle's parking brake system cannot adequately hold the vehicle stationary. Your parking brake isn't just a convenience—it's a safety system that regulators require to be fully functional and capable of keeping your truck in place when parked on level ground and on grades.
Parking brake defects range from mechanical failure (broken cables, worn friction surfaces, damaged linkage) to air brake system issues that prevent the parking brake from engaging or disengaging properly. The regulation doesn't care if you can still move the truck; it only cares whether the parking brake will actually hold it still.
This isn't about your service brakes (the ones you use while driving). It's specifically about the auxiliary system that's supposed to prevent rollaway when you're parked and not at the controls.
What our enforcement data actually shows
Across our 13 million inspection records, FMCSR 393.42 has generated 1,422 citations all-time, with 166 citations in the last 12 months and 26 in the last 90 days. This ranks the code at #627 of 3,036 FMCSR codes by citation volume—making it a less common violation than many brake and structural defects, but still a serious one when it does occur.
The critical number: our inspection records show a 64.9% out-of-service rate for 393.42. That means nearly two-thirds of vehicles cited for this defect are placed out of service on the spot. To put that in perspective, the all-FMCSR average OOS rate is 31.4%. This code triggers OOS decisions more than twice as often as the typical violation, making it one of the most enforcement-heavy violations an inspector will write.
Volume has climbed over the past year. The 12-month trend shows a spike from May through October (peaking at 24 citations in September), then dropping sharply in late 2025 before climbing again in early 2026. The spring surge suggests warmer weather and higher traffic volumes drive both inspection activity and detection rates.
Who gets cited most
Our data shows enforcement is concentrated in a handful of states. North Carolina leads with 17 citations in the last 180 days and the highest out-of-service rate at 94.1%—meaning inspectors there place nearly all cited vehicles out of service. Illinois follows with 11 citations but a 0.0% OOS rate, indicating a different enforcement posture. New Mexico has 10 citations with an 80.0% OOS rate, and Texas has 8 citations with a 75.0% rate.
The variation between NC (94.1% OOS) and IL (0.0% OOS) is striking. North Carolina inspectors are far more aggressive about removing vehicles; Illinois inspectors cite the defect but allow the vehicle to continue under certain conditions. If you operate in NC, understand that a 393.42 citation is very likely to end your trip immediately.
At the carrier level, our all-time data shows fleets such as Roadhouse (USDOT 4351464), Dealers Choice Truckaway System Inc (USDOT 255166), and Cochran Chemical Company Inc (USDOT 488136), each with 6 citations. This reflects the random distribution of mechanical failures across the industry rather than any pattern of negligence.
How severe is this compared to similar codes
In the Vehicle Maintenance category, 393.42 sits in the middle-to-high severity band. Compare it to nearby codes:
393.9(a) — Inoperable required lamps has logged 660,737 citations but only a 15.4% OOS rate. Lighting violations are far more common but far less likely to ground a truck.
396.3(a)(1) — Inspection/repair/maintenance (general) shows 236,919 citations with a 45.3% OOS rate. That's a much higher OOS rate than 393.42's 64.9%, but the raw volume is lower, suggesting inspectors enforce the general maintenance code broadly but reserve immediate out-of-service action for the worst cases.
393.47E — Slack adjuster defective has 180,363 citations but a 0.0% OOS rate, meaning parking brake defects are treated far more seriously than slack adjuster issues in actual enforcement.
Your 64.9% OOS rate makes 393.42 one of the highest-consequence violations in the brake and mechanical space. Only the most severe brake failures (like 20%+ of service brakes being defective) or critical structural issues see comparable enforcement action.
How to avoid it
Our inspection records reveal patterns in what co-occurs with 393.42 citations. The most common companion violation is 393.43 (brake relay emergency valve defect, appearing in 8 of the last 90 days' shared inspections). This tells us that parking brake failures often stem from air brake system problems, not just mechanical wear.
Here's what to do before you roll:
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Test your parking brake every morning. Apply it, try to gently move the truck forward in low gear (or neutral if you're sure you're safe). If it moves, the brake isn't holding. Don't leave the lot.
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Inspect air brake lines and fittings. Leaks in the parking brake circuit (fed by your air system) will prevent the brake from building or holding pressure. Look for loose fittings, kinks, or spray patterns around air lines.
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Check cable tension and condition on mechanical parking brakes. If your truck uses mechanical cables (wedge brakes or other designs), they should be tight, not frayed, and should move freely without binding. A cable that's visibly cracked or has severe rust will fail.
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Verify your parking brake fully releases. Just as important as setting is releasing. If you set the brake and then can't release it (or it releases partially), you have a defect that will be cited.
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Don't ignore the warning light or buzzer. Modern trucks have dash indicators for parking brake faults. If it lights up, get it checked before the next inspection.
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Know which vehicle makes need extra attention. Our data shows Ford vehicles (139 citations all-time) and Ram trucks (105 citations) appear most frequently in 393.42 violations. If you operate these platforms, budget for more frequent brake system inspection, especially the parking brake linkage and air valves.
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During pre-trip, don't assume the parking brake is fine because the truck is old. Wear and corrosion accelerate in older vehicles. Many of the vehicles in our violation data (Freightliner, Peterbilt, and trailer units) are workhorses that have logged hundreds of thousands of miles. Age is not an excuse; it's a reason to be more thorough.