49 CFR Part 393 is the equipment rulebook: it sets the minimum standards a commercial motor vehicle must meet to be on the road, covering brakes, lamps and reflectors, tires, wheels, mirrors, glazing, fuel systems, coupling devices, emergency equipment and cargo securement. Under 49 CFR 393.1, a motor carrier may not operate a CMV unless it is equipped as this part requires, and both the carrier and its employees must be knowledgeable of and comply with the specifications. Because these are pass/fail measurements taken at roadside, Part 393 produces more out-of-service orders than any other part of the safety regulations.
49 CFR Part 393 tells you what has to be on a commercial motor vehicle and what condition it has to be in. It sets minimum standards for brakes, lamps and reflectors, tires and wheels, mirrors, glazing, fuel systems, exhaust, coupling devices, emergency equipment, frames and suspensions, and — in its largest subpart — cargo securement.
It is the single most cited part of the Federal Motor Carrier Safety Regulations at roadside, for a structural reason: nearly every requirement is a measurable, pass/fail condition that an inspector can verify in a few minutes with a tread gauge, a flashlight and a pry bar. There is no judgment call and no explanation that fixes a 1/32-inch tire.
What does 49 CFR Part 393 actually require?
49 CFR 393.1 sets the frame. The rules establish minimum standards for commercial motor vehicles as defined in 49 CFR 390.5. “Every motor carrier and its employees must be knowledgeable of and comply with the requirements and specifications of this part,” and a motor carrier may not operate a CMV unless it is equipped in accordance with them. Where a requirement references vehicles with a GVWR below 10,001 pounds, it applies only when that vehicle actually meets the CMV definition. Intermodal equipment providers carry a parallel obligation for equipment they interchange to motor carriers.
The part is organized by system:
- Subpart B — lamps, reflective devices and electrical wiring (393.9–393.33)
- Subpart C — brakes (393.40–393.55)
- Subpart D — glazing and window construction (393.60–393.63)
- Subpart E — fuel systems (393.65–393.69)
- Subpart F — coupling devices and towing methods (393.70–393.71)
- Subpart G — miscellaneous parts and accessories (393.75–393.95)
- Subpart I — protection against shifting and falling cargo (393.100–393.136)
- Subpart J — frames, cab and body, wheels, steering and suspension (393.201–393.209)
A recurring pattern is worth understanding before you read any individual section: many requirements are stated as “must meet FMVSS No. X in effect on the date of manufacture.” Part 393 rarely re-specifies a component that the Federal Motor Vehicle Safety Standards already govern. It instead requires that the original safety-standard equipment be present and functional for the life of the vehicle, which is why a defeated or removed factory system is a 393 violation even though the truck was compliant when it was built.
What are the Part 393 numbers you have to know?
These are the thresholds that decide inspections. Every one is stated in the regulation itself.
| Item | The threshold | Citation |
|---|---|---|
| Tread depth, front wheels of a bus, truck or truck tractor | At least 4/32 inch in a major tread groove | 49 CFR 393.75(b) |
| Tread depth, all other tires | At least 2/32 inch in a major tread groove | 49 CFR 393.75(c) |
| Tire condition | No exposed body ply or belt material, no tread or sidewall separation, not flat, no audible leak | 49 CFR 393.75(a) |
| Regrooved, recapped or retreaded tires on bus front wheels | Prohibited | 49 CFR 393.75(d) |
| Regrooved tire on truck or truck tractor front wheels | Prohibited if load-carrying capacity is 2,232 kg (4,920 lb) or greater | 49 CFR 393.75(e) |
| Overloaded tire operation under a State special permit | Speed must not exceed 50 mph | 49 CFR 393.75 |
| Brake lining, steering axle, air drum brake with a continuous strip | Not less than 4.8 mm (3/16 inch) | 49 CFR 393.47(d) |
| Brake lining, steering axle, air drum brake with two pads | Not less than 6.4 mm (1/4 inch) | 49 CFR 393.47(d) |
| Brake lining, non-steering axle, air drum brake | Not less than 6.4 mm (1/4 inch) | 49 CFR 393.47(d) |
| Brake pad, air disc brake | Not less than 3.2 mm (1/8 inch) | 49 CFR 393.47(d) |
| Brake lining, hydraulic or electric brake | Not less than 1.6 mm (1/16 inch) | 49 CFR 393.47(d) |
| Low air pressure warning | Must warn at 379 kPa (55 psi) or below, or one-half of compressor governor cutout, whichever is less | 49 CFR 393.51 |
| Service brake stopping distance from 20 mph, most combinations | 40 feet, at a deceleration of 14 ft/sec² | 49 CFR 393.52 |
| Emergency brake stopping distance from 20 mph, most combinations | 90 feet | 49 CFR 393.52 |
| Windshield and driver’s side/left windows tint | Parallel luminous transmittance not less than 70 percent | 49 CFR 393.60 |
| Windshield damage allowed in the driver’s sight area | Damage coverable by a 19 mm (3/4 inch) disc, no closer than 76 mm (3 inches) to similar damage | 49 CFR 393.60 |
| Fire extinguisher, placarded hazmat power unit | UL rating of 10 B:C or more | 49 CFR 393.95 |
| Fire extinguisher, non-hazmat power unit | UL rating of 5 B:C or more, or two rated 4 B:C or more | 49 CFR 393.95 |
| Warning devices carried on the vehicle | Three bidirectional reflective triangles (FMVSS No. 125), or listed alternatives | 49 CFR 393.95 |
| Rear impact guard, ground clearance | Bottom of the guard not more than 76.2 cm (30 inches) above the ground | 49 CFR 393.86 |
| Rear impact guard, forward of the rear extremity | Not more than 61 cm (24 inches) | 49 CFR 393.86 |
| Rear impact guard, lateral position | Outermost surfaces not more than 45.7 cm (18 inches) from each side extremity | 49 CFR 393.86 |
| Warning flags on a projecting load | Required when the load projects more than 1,219 mm (4 feet) beyond the rear; flags at least 457 mm (18 inches) square, red or orange fluorescent | 49 CFR 393.87 |
| Air suspension leakage | Not more than 3 psi in 5 minutes at normal operating pressure | 49 CFR 393.207(f) |
| Steering wheel lash, 20-inch power steering wheel | Not more than 133 mm (5 1/4 inches) | 49 CFR 393.209 |
| Steering wheel lash, diameters not listed | Not more than 14 degrees manual, 30 degrees power | 49 CFR 393.209 |
| Sleeper berth minimum size, installed after Sept 30, 1975 | At least 75 inches long, 24 inches wide, 24 inches high | 49 CFR 393.76 |
What does Part 393 require for brakes?
49 CFR 393.40 states the principle: each CMV must have brakes adequate to stop and hold the vehicle or combination, and must have a service brake system, a parking brake system, and an emergency braking capability. Where those systems are interconnected, a failure in any part of one must still leave the vehicle with operative brakes.
Three sections carry the most weight in the field:
- 393.42 — brakes on all wheels. Every CMV must be equipped with brakes acting on all wheels, subject to narrow exceptions for pre-July 1980 three-axle trucks that meet 393.52, driveaway-towaway operations, light trailers of 3,000 pounds or less whose axle weight does not exceed 40 percent of the towing vehicle’s axle weights, and a handful of specialized units.
- 393.48 — brakes to be operative. All brakes with which a vehicle is equipped must at all times be capable of operating. Front-wheel braking-effort reduction devices are permitted only under specific conditions, and an automatic device may not reduce braking force by more than 50 percent.
- 393.47 — actuators, slack adjusters, linings and drums. This is where the lining thickness numbers in the table above come from, along with the requirement that brake components be properly installed and that air chambers on the same axle be the same size and type.
Antilock brakes are required by 49 CFR 393.55 based on manufacture date: air-braked truck tractors manufactured on or after March 1, 1997; other air-braked CMVs on or after March 1, 1998; and hydraulic-braked trucks and buses on or after March 1, 1999. The malfunction indicator lamp is part of the requirement, not an optional extra — a burned-out or disconnected ABS lamp is a violation of 393.55 even if the ABS itself works.
What does Part 393 require for cargo securement?
Subpart I was rewritten in 2002 to align with the North American Cargo Securement Standard, and it is now performance-based rather than prescriptive.
The outcome. 49 CFR 393.100 requires that cargo be loaded and secured to prevent it from leaking, spilling, blowing or falling from the vehicle, and to prevent shifting that could adversely affect the vehicle’s stability or maneuverability.
The performance criteria. 49 CFR 393.102 requires securement systems to withstand, without exceeding the manufacturer’s breaking strength rating, 0.8 g deceleration forward, 0.5 g acceleration rearward, and 0.5 g lateral acceleration. Measured against working load limits rather than breaking strength, the thresholds are 0.435 g forward, 0.5 g rearward and 0.25 g lateral. Cargo not fully contained within the vehicle structure must also receive a downward force of at least 20 percent of its weight.
The devices. 49 CFR 393.104 requires all tiedowns, systems, parts and components to be in proper working order with no damaged or weakened components such as cracks or cuts that will adversely affect performance, and states flatly that tiedowns and securing devices “must not contain knots.”
The math. 49 CFR 393.106 requires the aggregate working load limit of the tiedowns to be at least one-half times the weight of the article or group of articles secured. A tiedown attached from the vehicle to an anchor point on the cargo counts as half its working load limit; a tiedown that runs from an anchor point on one side of the vehicle, through or over the cargo, to an anchor point on the other side counts at full working load limit.
The count. 49 CFR 393.110 sets minimum tiedown numbers by article length and weight, as summarized in the FAQ above. Sections 393.116 through 393.136 then add commodity-specific rules for logs, dressed lumber, metal coils, paper rolls, concrete pipe, intermodal containers, vehicles, flattened cars, roll-on/roll-off containers and large boulders.
The driver-side counterpart lives in Part 392: 49 CFR 392.9 requires the first securement check within 50 miles and re-examination at every duty-status change, or every 3 hours, or every 150 miles — whichever comes first.
What do inspectors actually check under Part 393?
A North American Standard Level I inspection walks the vehicle in a fixed order, and Part 393 supplies most of the checklist.
- Lamps. Every required lamp is switched on and observed. 49 CFR 393.9 requires that all lamps required by Subpart B be capable of being operated at all times and not obscured by the tailboard, load, covering, dirt or added equipment.
- Tires. A gauge goes into a major tread groove on the steer axle first — 4/32 — then the drives and trailer at 2/32. Sidewalls are checked for exposed cord, separation and cuts. Inflation is checked by gauge, not by tapping.
- Brakes. Pushrod stroke is measured against the chamber’s adjustment limit, linings are measured against the 393.47(d) thresholds, air loss is timed, and the low-pressure warning is verified against the 55 psi threshold in 393.51.
- Wheels and suspension. 49 CFR 393.205 prohibits cracked or broken wheels and rims, elongated stud or bolt holes, and missing or loose nuts or bolts. 49 CFR 393.207 prohibits cracked, broken, loose or missing axle positioning parts, shifted or broken leaf springs, and air leakage above 3 psi in five minutes.
- Coupling. 49 CFR 393.70 requires the fifth wheel lower half to be secured to the frame with properly designed brackets and properly tightened bolts, and requires a locking mechanism that prevents separation unless a positive manual release is activated. Full trailers and converter dollies require safety devices to the towing vehicle’s frame.
- Frame, glazing, exhaust, mirrors. 49 CFR 393.201 prohibits cracked, loose, sagging or broken frames and unauthorized holes in the rail flanges. 49 CFR 393.83 prohibits exhaust leaks or discharge forward of or directly below the driver/sleeper compartment and prohibits temporary repair “with wrap or patches.” 49 CFR 393.80 requires two rear-vision mirrors, one on each side.
- Emergency equipment. The box is opened and counted: extinguisher present, mounted, charged and correctly rated; emergency reflective triangles present and conforming; (the separate spare-fuse requirement was removed effective April 20, 2026 — see the regulatory changes tracker); three triangles or an approved alternative.
In an office-based compliance review the emphasis shifts. The auditor is not measuring linings; they are asking whether your maintenance records under Part 396 show that defects found at roadside were repaired, and whether the same unit keeps producing the same 393 violation. Carriers that keep unit-level maintenance history, inspection results and document expirations in one system — the workflow behind fleet management software — can produce that history on demand rather than assembling it from shop invoices.
What are the most common Part 393 violations?
FMCSA roadside inspection data has been remarkably stable on this point for years. The recurring leaders are:
- Inoperative required lamp, 49 CFR 393.9. Year after year this is at or near the top of the vehicle violation list. Lamps sit in the weather, take road debris and depend on connectors and grounds that corrode.
- Brakes out of adjustment and defective brakes, 49 CFR 393.47 and 393.48. These carry high severity weights in the Vehicle Maintenance BASIC and drive a large share of out-of-service orders.
- Tires, 49 CFR 393.75. Flat tires and audible air leaks, then tread depth below 2/32.
- Cargo securement, 49 CFR 393.100 through 393.136. Insufficient tiedowns, damaged straps, and failure to meet the aggregate working load limit.
- Oil and grease leaks and exhaust defects, 49 CFR 393.83 and related sections.
- Wheel and rim defects, 49 CFR 393.205, typically loose or missing fasteners found during the walk-around.
The pattern is worth naming: brakes, tires and lights account for the majority of vehicle out-of-service orders, and all three are conditions a competent pre-trip inspection catches. A truck placed out of service for a lamp was almost always a truck nobody walked around that morning.
How do you stay ahead of Part 393?
Three habits do most of the work.
Measure, don’t eyeball. Put a tread depth gauge and a lining gauge in every truck and in the shop. The difference between 3/32 and 4/32 on a steer tire is invisible and decisive.
Fix the repeat offender, not the ticket. If the same trailer produces a 393.9 violation three times in a year, the problem is a wiring harness or a ground, not three bulbs. Track violations by unit, not just by date.
Close the loop from roadside to shop. Every inspection report with a Part 393 violation should generate a repair record, a signed certification of repair under Part 396, and a return-to-service date. That paper trail is what an auditor asks for, and it is the difference between one bad inspection and a pattern of neglect.
For how Part 393 interacts with the rest of the safety regulations, see the DOT compliance guide, the hazmat-specific driving and parking rules in Part 397, and the full 49 CFR reference library. Fleets tracking equipment defects and inspection outcomes across a whole yard usually end up building it into their compliance and safety workflow rather than a spreadsheet, and that is also the core of fleet compliance tracking.
This page is a plain-English summary of 49 CFR Part 393 and is not legal advice. Always read the current regulation text and consult qualified counsel for your specific operation.
Note: This is a plain-English summary of 49 CFR Part 393, current as of the date above, and is general information rather than legal advice. The regulation itself is controlling — read it on the eCFR and confirm current requirements with the FMCSA.