Parts and Accessories for Safe Operation

Parts and Accessories Necessary for Safe Operation is the FMCSA regulation that sets minimum equipment standards for commercial motor vehicles (CMVs). Found in 49 CFR Part 393, it covers lamps and reflective devices, brakes, glazing and windshields, fuel systems, coupling devices, tires, mirrors, emergency equipment, frames, steering, suspension and cargo securement. If a required part is missing, broken or out of adjustment, the vehicle does not comply.

Part 393 is the rulebook behind most roadside vehicle violations. Inspectors check vehicles against these standards, and the North American Standard Out-of-Service Criteria identify the defects serious enough to stop a vehicle on the spot. Fleets that build Part 393 into their maintenance and pre-trip checks see fewer out-of-service orders, better Vehicle Maintenance scores and fewer breakdowns.

What Is Part 393?

49 CFR Part 393 is organized into subparts by system. Under 393.1, no employer may operate a CMV, and no employee may drive one, unless it is equipped as Part 393 requires. A companion rule in 396.3 requires all parts and accessories to be in safe and proper operating condition at all times. Part 393 says what equipment must be on the vehicle, and the inspection, repair and maintenance rules in Part 396 say how carriers must keep it working.

Many Part 393 requirements mirror the Federal Motor Vehicle Safety Standards (FMVSS) that manufacturers must meet when vehicles are built. Part 393 carries those standards into the vehicle's working life, and some requirements depend on the date the vehicle was manufactured.

Who Must Comply?

Part 393 applies to motor carriers operating CMVs in interstate commerce and to their drivers. This includes trucks, truck tractors, trailers, semitrailers and buses that meet the CMV definition in 390.5, such as vehicles with a gross vehicle weight rating of 10,001 pounds or more. Most states adopt Part 393 or compatible standards for intrastate vehicles through the Motor Carrier Safety Assistance Program.

Key Requirements

Lamps and reflective devices (Subpart B)

Section 393.11 lists the required lamps, reflectors and reflective material, and 393.9 requires every required lamp to be capable of operating at all times. Key points:

• Headlamps, turn signals, stop lamps, tail lamps, clearance and identification lamps, and side marker lamps and reflectors are required according to vehicle type and size.
• Trailers 80 inches or more wide with a GVWR of 10,001 pounds or more manufactured on or after December 1, 1993 must have retroreflective conspicuity treatment, typically red and white sheeting or tape. Older trailers of that size had to be retrofitted.
• Truck tractors manufactured on or after July 1, 1997 must also have conspicuity treatment.
• Reflective sheeting must be kept clean and in place to be effective.

Brakes (Subpart C)

• Every CMV must have service brakes, a parking brake system and, for towing vehicles, a means of applying trailer brakes in an emergency (393.40 through 393.43).
• All brakes must be operative under 393.48, with limited exceptions such as towed vehicles in driveaway-towaway operations.
• Air-braked vehicles need a low-air warning device (393.51).
• Air-braked CMVs manufactured on or after October 20, 1994, and hydraulic-braked CMVs manufactured on or after October 20, 1993, must have automatic brake adjusters (393.53).
• Under 393.55, antilock brake systems are required on air-braked truck tractors manufactured on or after March 1, 1997, other air-braked CMVs manufactured on or after March 1, 1998, and hydraulic-braked trucks and buses manufactured on or after March 1, 1999, along with ABS malfunction indicator lamps.

Brakes out of adjustment are consistently among the most common vehicle out-of-service conditions at roadside.

Glazing and windshields (Subpart D)

Under 393.60, the windshield must be free of discoloration or damage in the critical viewing area swept by the wipers, with limited exceptions for small chips and cracks that meet size limits. Windshield wipers (393.78) and defrosting devices (393.79) must work, and the rule limits what may be mounted on or tinted in the windshield.

Tires (393.75)

• Tires may not have exposed ply or belt material, tread or sidewall separation, a flat or audible leak, or a cut exposing the belt.
• Tread depth must be at least 4/32 inch in every major tread groove on tires on the front steering axle, and at least 2/32 inch on all other tires.
• Buses may not use regrooved, recapped or retreaded tires on the front wheels.
• Tires must not be loaded beyond their rated capacity.

Coupling devices and towing (Subpart F)

Fifth wheels, kingpins, pintle hooks, drawbars and towbars must be securely mounted and in good condition under 393.70 and 393.71. Full trailers and converter dollies must be connected with safety chains or cables strong enough to keep the trailer attached if the main connection fails.

Other vehicle systems

• Mirrors (393.80): mirrors must give the driver a view of the highway to the rear along both sides of the vehicle.
• Horn and speedometer (393.81, 393.82): both must work.
• Exhaust (393.83): no leaks or discharge where fumes could enter the cab.
• Rear impact guards (393.86): required on most trailers and semitrailers, with dimensional and strength requirements.
• Seat belts (393.93): most CMVs must have a seat belt assembly at the driver's seat.
• Frames, cabs, wheels, suspension and steering (Subpart G): no cracked, broken or loose frame members, no loose or missing wheel fasteners, no broken springs and no excessive steering wheel lash.

Emergency Equipment (393.95)

Trucks, truck tractors and buses, other than vehicles towed in driveaway-towaway operations, must carry:

• A fire extinguisher: at least one rated 5 B:C or two rated 4 B:C on non-hazmat vehicles, or one rated 10 B:C or more on vehicles that must be placarded for hazardous materials. Extinguishers must be securely mounted, readily accessible, show a UL rating and have a gauge or other means to confirm they are fully charged.
• Warning devices: three bidirectional emergency reflective triangles, or at least six fusees as an alternative, subject to restrictions on flame-producing devices for vehicles carrying certain explosives and flammables. Section 392.22 explains when and where to place them after a stop.

FMCSA removed the requirement to carry spare fuses in a final rule published in February 2026, along with several other Part 393 updates. Always check the current eCFR text before updating checklists.

Cargo Securement (Subpart I)

Sections 393.100 through 393.136 set performance criteria, working load limits, tiedown counts and commodity-specific rules for logs, metal coils, heavy equipment, concrete pipe and other loads. See the cargo securement page for details.

Penalties and Enforcement

Part 393 violations are cited during roadside inspections, compliance investigations and post-crash reviews. Defects that meet the CVSA North American Standard Out-of-Service Criteria, which are updated each April, result in the vehicle being placed out of service until repaired. Violations count in the Vehicle Maintenance BASIC of the CSA Safety Measurement System, and the Vehicle factor in a safety rating considers out-of-service rates and Parts 393 and 396 compliance. Civil penalties are adjusted annually for inflation.

Common Compliance Mistakes

• Brakes out of adjustment because slack adjusters are not checked during routine service
• Inoperative lamps, especially clearance and marker lamps on trailers
• Worn steer tires approaching the 4/32-inch limit
• Discharged, unmounted or missing fire extinguishers and warning triangles
• Faded or missing conspicuity tape on trailers
• Cracked windshields in the wiper area left unrepaired

How to Comply: Step by Step

1. Know your equipment. Record each vehicle's manufacture date so you know which date-based requirements apply.
2. Build Part 393 into inspections. Align pre-trip checklists, driver vehicle inspection reports and preventive maintenance forms with Part 393.
3. Check brakes regularly. Measure pushrod stroke and inspect brake components at each service interval.
4. Monitor tires and lamps. Measure tread depth and test every lamp during inspections.
5. Stock emergency equipment. Confirm each power unit has a charged, mounted extinguisher and three triangles.
6. Repair before dispatch. Do not dispatch vehicles with safety defects, and document every repair.
7. Review roadside results. Track violations by system and fix recurring problems at the source.

How SMS360 Helps

SMS360's Vehicle Management module keeps vehicle records, inspection checklists, defects and repair history in one place, with reminders for scheduled maintenance. Audits & Inspections supports mobile equipment checklists with photos, and Actions Management assigns and tracks repairs to closure.

Frequently Asked Questions

What is the minimum tread depth for truck tires?

Tires on the front steering axle need at least 4/32 inch of tread in every major groove. All other tires need at least 2/32 inch.

What fire extinguisher does a CMV need?

Non-hazmat power units need one extinguisher rated 5 B:C or two rated 4 B:C. Vehicles that must be placarded for hazardous materials need one rated 10 B:C or more.

Are spare fuses still required?

No. FMCSA removed the spare fuse requirement from 393.95 in a final rule published in February 2026.

Which trailers need reflective tape?

Trailers 80 inches or more wide with a GVWR of 10,001 pounds or more need conspicuity treatment under 393.11. Truck tractors manufactured on or after July 1, 1997 also need it.