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CDL Study Guides · 2026
The pre-trip inspection is both a federal requirement before every shift and one of the first things you're graded on during the CDL skills test. Knowing it cold protects your license and your load.
Last reviewed 2026-06-20 · Based on FMCSA standards (see sources)
The engine compartment inspection is typically the first step in your pre-trip. Before opening the hood, check underneath the vehicle for fresh leaks — oil, coolant, fuel, or hydraulic fluid. Any fresh leak is a potential problem that must be addressed.
With the hood open, start with fluid levels. Check the engine oil using the dipstick — the level should be between the ADD and FULL marks. Check the coolant level in the overflow tank or radiator (only open a radiator cap when the engine is cool). Check power steering fluid in the reservoir. Check the windshield washer fluid level.
Inspect all belts and hoses. Belts should have no more than about 3/4 inch of play (slack) and should not be frayed, cracked, or glazed. Hoses should be firm but flexible, with no bulges, cracks, or leaks at the connections. Pay special attention to the hose clamps — loose clamps cause leaks.
Check the air compressor and its drive mechanism. If belt-driven, the belt condition and tension are important. If gear-driven, look for oil leaks around the compressor mount.
Inspect the wiring and electrical connections. Look for frayed, cracked, or disconnected wires. Battery connections should be clean, tight, and free of corrosion. The battery should be secured in its mount — a loose battery can short out and cause a fire. Check the alternator mounting and connections.
The steering system begins in the engine compartment. Check the power steering pump for leaks, proper belt tension, and fluid level. Look at the steering gear box for leaks and proper mounting. Trace the steering linkage from the steering box to the wheels — look for loose or missing cotter pins, worn or cracked joints, and excessive play.
Before closing the hood, look at the frame and crossmembers. Check for cracks, rust-through, or missing bolts. Look at the suspension mounting points for damage or loose hardware.
Key points to remember
The cab inspection ensures that all driver controls, gauges, and safety devices are working properly before you start driving.
Start with the seat and mirrors. Adjust the seat so you can reach all controls and have a clear view of the road and mirrors. Adjust all mirrors — you should be able to see down both sides of the vehicle and the area immediately behind the trailer. Fasten your seat belt.
Check the instrument panel. All gauges should be working and showing normal readings. Specifically check: oil pressure gauge (should show normal pressure with the engine running), ammeter or voltmeter (should show the battery is charging), coolant temperature gauge (should not be in the red), air pressure gauge (should show at least 100 psi in both the primary and secondary systems), and fuel level.
Test all controls. Check the parking brake — it should hold the vehicle on flat ground. Check the service brake pedal — it should not feel spongy or go to the floor. Check the clutch for proper free play. Test the horn, windshield wipers, windshield washers, heater, defroster, and all lights (headlights on both high and low beam, turn signals, four-way flashers).
Emergency equipment in the cab includes the fire extinguisher (must be properly charged and easily accessible — check the gauge and inspection tag), warning triangles (three required), and spare electrical fuses (unless the vehicle has circuit breakers). The cab should also have a means of communication — a CB radio, cell phone, or dispatch radio.
Check the cab structure itself. Doors should open, close, and latch properly. Windows should raise and lower. The windshield should be free of illegal cracks (any crack in the wiper zone on the driver's side, or intersecting cracks). The sun visor should be functional.
On the floor, check for loose items that could roll under the brake or clutch pedal. Nothing should obstruct the pedals or prevent you from reaching them quickly.
Key points to remember
The wheel and brake inspection is one of the most important parts of the pre-trip because brake and tire failures are among the leading causes of commercial vehicle accidents.
Start with the tires. Check every tire on every axle. Look for proper inflation — under-inflated or over-inflated tires are dangerous. Check tread depth: steer tires must have at least 4/32 inch of tread, and all other tires must have at least 2/32 inch. Look for cuts, bulges, uneven wear, foreign objects, and exposed cords or belts. Dual tires should not be touching each other — a gap should be visible between them.
Check the wheels and rims. Look for cracks, dents, or distortion. Missing or damaged lug nuts mean the vehicle should not be driven — all lug nuts must be present and tight. Rust trails from lug nuts indicate they are loose. Check for proper wheel spacer condition on dual wheel assemblies.
Inspect the hub oil/grease seals. Oil hub wheels should have oil visible in the sight glass at the proper level. Grease seals should not be leaking. Excessive grease around the seal area indicates a failing seal.
The brake inspection is critical. Look through the inspection port on each brake drum. Brake linings must be at least 1/4 inch thick. If you cannot see the linings, the drums should be removed for inspection (this is typically done at a shop, not on the road). Check brake drums for cracks and excessive wear.
Check the brake hoses and lines. They should be securely mounted, not rubbing against anything, and free of leaks. Air brake chambers should not be cracked or dented. The push rod should not be stroked out more than its specified limit — if it is, the slack adjuster needs attention.
The ABS (Anti-lock Braking System) warning light should illuminate briefly when you turn on the ignition and then turn off. If it stays on, the ABS has a fault. Note: the base brakes still work — only the anti-lock function is affected. But a faulty ABS should be repaired as soon as possible.
Check the suspension components while you are looking at the wheels and brakes. Look for broken, missing, or shifted leaf springs; damaged spring hangers; cracked or leaking air bags (on air-ride suspensions); and damaged shock absorbers.
Key points to remember
The external walk-around is your chance to see the full picture of the vehicle before driving. Walk completely around the vehicle, inspecting every component you can see.
Start at the front of the vehicle. Check the headlights (both low and high beam) for proper operation, cracks, and cleanliness. Check turn signals and clearance lights. Inspect the front bumper for damage. Look at the license plate and any required stickers or permits.
Move along the driver's side. Check the fuel tank for leaks, secure mounting, and a tight cap. Check the battery box if it is on this side — the box should be secure and the batteries should not be leaking. Inspect the exhaust system: look for leaks from manifold to tailpipe. The exhaust must not contact fuel lines or tires.
Check the frame and body for damage. Look under the vehicle for anything hanging loose or dragging. Check the spare tire mount and condition if applicable.
At the rear, check all lights: brake lights, turn signals, clearance lights, and reflectors. Reflectors must be clean and not cracked. The rear bumper must be present and secure. Check the rear license plate and light.
Continue along the passenger side, checking the same items as the driver's side. Pay particular attention to items that may be harder to see from the cab, such as tire condition on the right side.
For the lighting check, you will need to walk around the vehicle multiple times or have someone help you verify that all lights are working. Start with parking lights and clearance lights, then check low beams, high beams, turn signals (left and right), four-way flashers, and brake lights. A burned-out light can result in a citation and creates a safety hazard.
Finally, make sure all cargo doors or trailer doors are secure and properly latched. Check that mud flaps are in place and not torn or missing. Verify that all mirrors are clean, properly adjusted, and not cracked.
Key points to remember
Run real practice questions on this section until you're comfortably past 80%.
Practice Pre-Trip Inspection questionsSteer-axle tires must have at least 4/32 of an inch of tread; all other tires need at least 2/32 of an inch. You're also checking for cuts, bulges, and objects stuck in the tread, and that tires are evenly inflated with no signs of a slow leak.
On a vehicle with power steering, the steering wheel should have no more than about 2 inches (roughly 10 degrees) of free play before the front wheels begin to move. More than that is a defect and the vehicle shouldn't be driven until it's fixed.
Rust streaks running outward from the wheel lug nuts are a classic sign the nuts are loose. For brakes, you're checking that linings aren't worn past about 1/4 inch, that hoses and chambers aren't leaking or cracked, and listening for the hiss of an air leak. Any missing or broken lug nut means the truck is out of service.
The examiner expects you to walk the vehicle and name what you're checking and why — pointing out that a part is present, secure, and not leaking, worn, or cracked. It's the first graded portion of the skills test, so practicing the walk-around out loud is one of the highest-value things you can do before test day.
See also CDL costs by state · Your state's rules · Sources
Based on the FMCSA CDL Manual, a U.S. federal government publication in the public domain. Independent study aid — not affiliated with or endorsed by FMCSA, AAMVA, or any state DMV. Always confirm current rules with your state DMV.