CDL knowledge test

Combination Vehicles practice test

Required for Class A. Covers coupling and uncoupling, fifth wheel inspection, rollover risk, off-tracking, and trailer control.

58 questions available. A study sitting draws 20, and the App practice target: 80%; this is not an official test score. Your state sets the real test length.

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Combination Vehicles questions with answers

Every question below carries the answer, the reason, and the section of the FMCSA CDL Manual or 49 CFR it comes from. Nothing here is a recalled exam question.

For a conventional fifth wheel, which preparation matches the coupling procedure?
- ARear edge tilted up and jaws closed
- BRear edge tilted down and jaws openCorrect
- CFifth wheel turned sideways to the kingpin
- DSliding fifth wheel left unlocked
Prepare the fifth wheel to receive the kingpin: rear edge down, jaws open, and any slide locked. A closed or misaligned connection can prevent proper engagement.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
After coupling, when you test the connection by pulling forward, you should:
- APull gently in low gearCorrect
- BRelease the trailer brakes first
- CNot pull at all—just check visually
- DPull firmly to ensure the kingpin doesn't pop out
With the trailer brakes locked and landing gear just clear of the ground, pull gently in low gear. Then secure the tractor and visually inspect the coupling. High force can damage equipment; resistance alone does not show exactly where the jaws engaged.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
Landing gear is already down. You have uncoupled and stopped with the tractor frame still under the trailer. What comes before pulling fully clear?
- ADrive away without another check
- BReconnect the fifth wheel without inspecting
- CRaise the trailer supports off the ground
- DSecure the tractor and inspect the landing gear and supporting groundCorrect
Keep the frame underneath while you secure the tractor and check that the trailer supports and ground are holding. This pause limits the risk of a drop if the supports sink or fail.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
Which action provides vertical support for the front of a semitrailer before the tractor is removed during uncoupling?
- ALower the landing gearCorrect
- BRemove the air lines
- CDisconnect the electrical cord
- DRelease the parking brake
Lower the trailer landing gear onto sound supporting ground before releasing the coupling. Air lines, the electrical cord, and a parking brake do not provide vertical support for the trailer nose.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
What is a 'converter dolly' used for?
- ATo convert a flatbed to a refrigerated trailer
- BTo connect a second semitrailer to a lead trailer, forming a double combinationCorrect
- CTo connect a full trailer to a tractor
- DTo convert metric axle weights to U.S. standards
A converter dolly provides axles and a fifth wheel so another semitrailer can follow the lead trailer. It is a separate coupling unit, not a device for refrigeration, axle measurement, or changing a trailer body.
Oregon DMV Commercial Driver Manual · 7.2 Coupling and Uncoupling; 7.3 Inspection; 7.4 Brake Checks · verified 2026-09-07
When coupling a tractor to a semitrailer, after connecting the air lines you should:
- ASkip the brake checks if the hoses look connected
- BMove the trailer immediately to test it
- CDisconnect the trailer air supply and leave the checks unfinished
- DSupply air and check trailer brake operation and major air loss before the next coupling stepCorrect
Charge to normal pressure and check brake response, crossed lines, and major air loss with the vehicle secured. The mechanical engagement, gentle connection check, and visual inspection come later in this conventional procedure.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
When trailer air-line couplings use the standard red/blue coding, which pairing is correct?
- ABlue for service brakes, red for emergency (supply) brakesCorrect
- BRed for service, blue for emergency
- CYellow for service, green for emergency
- DBoth couplings are the same color
Red identifies the emergency/supply connection and blue the service connection. Check markings and actual connections; crossed lines can prevent proper brake operation. Do not charge a system with known mismatched lines.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
During the initial approach, before checking trailer height and backing fully underneath, how should you position the tractor?
- AAt a high speed to seat the kingpin
- BWithout aligning the fifth wheel
- CSlowly, straight back, until the fifth wheel just touches the trailerCorrect
- DWhile turning sharply to align with the trailer
Approach straight and slowly until the fifth wheel just contacts the trailer. Secure the tractor and check height and alignment before the later engagement step. Initial contact alone is not a completed coupling.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
After coupling, the 'tug test' confirms the connection by:
- APushing the trailer backward with the tractor at low speed
- BGently pulling forward with the trailer brakes locked to check it holdsCorrect
- CLifting the front of the trailer with a jack
- DRevving the engine while the trailer brakes are holding
A gentle pull against locked trailer brakes checks that the connection resists separation. Secure the tractor and also inspect for no plate gap, jaws around the kingpin shank, and a secured locking mechanism. The tug does not replace those visual checks.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
When uncoupling a loaded semitrailer with conventional landing gear, how far should you lower the supports?
- AOnly after the tractor is fully clear
- BLeave the supports off the ground
- CTo firm ground contact, then a few low-gear turns to relieve some tractor weight without lifting the trailer off the fifth wheelCorrect
- DDo not lower them because the trailer is loaded
For a loaded trailer, firm ground contact plus a few low-gear turns removes some weight from the tractor. Do not jack the trailer off the fifth wheel. An empty trailer needs firm support without the loaded-trailer extra-turn assumption.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
Before supplying trailer air, you see the blue service hose attached to the red supply connection. What should you do?
- AKeep the vehicle secured and correct the mismatched connections before chargingCorrect
- BSupply air first to see whether the mismatch matters
- CConnect only one hose and leave the other loose
- DRelease the trailer brakes and begin backing
Identify and connect each line to its matching function before supplying air. Then check operation and leakage as prescribed. Do not use an incorrect connection to release brakes or move the trailer.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
What is the correct order of steps when uncoupling a tractor-semitrailer?
- ASecure the rig and chock as required → lower trailer landing gear → disconnect lines → unlock → pull partly clear → secure and inspect supports → pull fully clearCorrect
- BDisconnect lines → remove the tractor → lower landing gear
- CChock wheels → remove the tractor → lower landing gear
- DLower landing gear → remove the tractor without releasing or checking the coupling
Establish brake/chock restraint and vertical support before separation. Stop with the tractor frame still under the trailer and check the supports before leaving. Chocks are not a final step after the trailer is already unsupported.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
What does the 'tug test' check when coupling?
- AThat the air lines are properly connected
- BThat the fifth wheel jaws are locked around the kingpinCorrect
- CThat the landing gear has been raised
- DThat the trailer tires are properly inflated
A gentle pull against locked trailer brakes checks that the connection resists separation. Secure the tractor and also inspect for no plate gap, jaws around the kingpin shank, and a secured locking mechanism. The tug does not replace those visual checks.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
Under 49 CFR 390.5T, which statement describes a full trailer?
- ANo part of its weight, except the towing device, rests on the self-propelled towing motor vehicleCorrect
- BIt is loaded to its gross vehicle weight rating
- CIt is sealed from weather on all sides
- DIt has been certified for highway use
For these federal definitions, a full trailer does not rest on the towing motor vehicle except at the towing device. A semitrailer is constructed so that some part of its weight rests on the towing motor vehicle.
Electronic Code of Federal Regulations · 49 CFR 390.5T, Trailer definitions · verified 2026-09-07
A combination vehicle cannot complete a right turn without entering another traffic lane. What is the safer turn method?
- ASwing left before starting the turn and leave the right side open
- BKeep the rear close enough to block a right-side pass, then turn wide while completing the turnCorrect
- CLet the trailer wheels cross the curb if traffic is present
- DStop diagonally across both lanes before steering
Off-tracking makes the trailer wheels cut inside the tractor path. Keep the rear close enough to prevent a vehicle from passing on the right, and move wide as you complete the turn. Swinging left before the turn opens a dangerous gap on the right.
FMCSA Commercial Driver's License Manual · Section 6.1.6: Turn Wide · verified 2026-09-08
What must a driver do before routing a combination vehicle near low bridges or other overhead obstructions?
- AKnow the actual loaded height and check route clearancesCorrect
- BAssume every U.S. highway allows a 13-foot-6-inch vehicle
- CUse only the posted passenger-car speed limit
- DRely on the trailer color to determine clearance
There is no single federal commercial-motor-vehicle height limit. State and route limits vary. Before travel, know the actual loaded height and compare it with posted clearances and the permitted route; do not rely on a common height figure as a national rule.
FHWA — Federal Size Regulations for Commercial Motor Vehicles · Vehicle height — no federal CMV height requirement; state limits vary · verified 2026-09-07
Under 49 CFR 383.91, which combination is in the Class A CDL category?
- AA combination with a GCWR of 26,001 pounds or more, towing a unit with GVWR over 10,000 poundsCorrect
- BAny vehicle towing a trailer with GVWR over 10,000 pounds, regardless of combination weight
- CAny vehicle requiring a tanker endorsement
- DAny school bus towing a trailer
Under 49 CFR 383.91(a)(1), Class A covers any combination with a gross combination weight rating (GCWR) of 26,001 pounds or more when the towed vehicle or vehicles have a gross vehicle weight rating (GVWR) over 10,000 pounds. Endorsements can add separate requirements.
eCFR 49 CFR 383.91 — Commercial driver's license classes and endorsements · 383.91(a) — Class A CDL · verified 2026-09-23
What connects a tractor to a semi-trailer?
- AThe pintle hook and eye
- BThe fifth wheel and kingpinCorrect
- CThe converter dolly tongue
- DThe landing gear brace
The trailer kingpin engages the tractor fifth wheel. The connection supports the trailer front and transmits forces between units; the trailer axles support the rear. Check the coupling before travel.
Oregon DMV Commercial Driver Manual · 6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling · verified 2026-09-07
What is the purpose of the tractor protection valve in a combination vehicle?
- ARaises the trailer's landing gear
- BApplies the trailer brakes at every stop
- CKeeps air in the tractor if the trailer breaks awayCorrect
- DLocks the fifth wheel in position
The tractor protection valve keeps air in the tractor or truck brake system if the trailer breaks away or develops a bad leak. When it closes, it stops air from leaving the tractor and vents the trailer emergency line, which applies the trailer emergency brakes.
FMCSA Commercial Driver License Manual · Section 6.2.2: Tractor Protection Valve · verified 2026-09-08
According to the FMCSA model CDL manual, what can happen if the service and emergency air lines are crossed?
- AYou could drive away without trailer brakesCorrect
- BThe trailer brakes will drag continuously
- CThe tractor will lose its parking brake
- DThe trailer lights will stop working
The FMCSA model manual says crossed lines can send supply air to the service line instead of charging trailer tanks. On older trailers, you could drive away without trailer brakes. Test trailer brakes before driving.
FMCSA Model Commercial Driver License Manual · Section 6.2: Air-line connections and trailer brakes · verified 2026-09-07

What this test is drawn from

2.18.5 ABS on the Tractor Only or Only on the Trailer
31113(a)–(c) — National Network width limitation and permits
383.91(a) — Class A CDL
49 CFR 390.5T, Trailer definitions
6.2.4 Air-lines; 6.2.5 Hose Couplers; 6.4.1 Coupling; 6.4.2 Uncoupling
6.5.1 Combination walk-around inspection; 6.5.2 brake checks
7.2 Coupling and Uncoupling; 7.3 Inspection; 7.4 Brake Checks
Section 11.2.2: Suspension
Section 11.2.6: Rear of Vehicle (Splash Guards); Section 2.1.5: Rear of Vehicle
Section 11.4.2: Side of Trailer (Tandem Release Arm/Locking Pins)
Section 2.2.4: Backing Safely
Section 2.6.1: Speed and Stopping Distance
Section 2.7.3: Space to the Sides
Section 2.7.7: Space Needed to Cross or Enter Traffic
Section 6 Coupling, Step 1 Inspect Fifth Wheel
Section 6.1.1: Rollover Risks
Section 6.1.2: Steer Gently
Section 6.1.5: Prevent Trailer Skids
Section 6.1.6: Turn Wide
Section 6.1.7: Backing with a Trailer
Section 6.2.2: Tractor Protection Valve
Section 6.2.3: Trailer Air Supply Control
Section 6.2.5: Hose Couplers (Glad Hands)
Section 6.2: Air-line connections and trailer brakes
Section 6.4.1: Coupling Tractor-Semitrailers, Step 14
Section 6.4.1: Coupling Tractor-Semitrailers, Step 15; Section 11.4.2: Side of Trailer (Landing Gear)
Section 6.4.1: Coupling Tractor-Semitrailers, Steps 7 and 14; Section 6.5.1: Inspecting Combination Vehicles
Section 6.4.2: Uncoupling Tractor-Semitrailers
Section 6.5.1: Coupling System Areas
Section 6.5.1: Sliding Fifth Wheel
Section 6.5.2: Test Trailer Emergency Brakes
Sections 6.4.1 and 6.5.1: Air and Electrical Lines
Vehicle height — no federal CMV height requirement; state limits vary

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Combination Vehicles practice test

Combination Vehicles questions with answers

For a conventional fifth wheel, which preparation matches the coupling procedure?

After coupling, when you test the connection by pulling forward, you should:

Landing gear is already down. You have uncoupled and stopped with the tractor frame still under the trailer. What comes before pulling fully clear?

Which action provides vertical support for the front of a semitrailer before the tractor is removed during uncoupling?

What is a 'converter dolly' used for?

When coupling a tractor to a semitrailer, after connecting the air lines you should:

When trailer air-line couplings use the standard red/blue coding, which pairing is correct?

During the initial approach, before checking trailer height and backing fully underneath, how should you position the tractor?

After coupling, the 'tug test' confirms the connection by:

When uncoupling a loaded semitrailer with conventional landing gear, how far should you lower the supports?

Before supplying trailer air, you see the blue service hose attached to the red supply connection. What should you do?

What is the correct order of steps when uncoupling a tractor-semitrailer?

What does the 'tug test' check when coupling?

Under 49 CFR 390.5T, which statement describes a full trailer?

A combination vehicle cannot complete a right turn without entering another traffic lane. What is the safer turn method?

What must a driver do before routing a combination vehicle near low bridges or other overhead obstructions?

Under 49 CFR 383.91, which combination is in the Class A CDL category?

What connects a tractor to a semi-trailer?

What is the purpose of the tractor protection valve in a combination vehicle?

According to the FMCSA model CDL manual, what can happen if the service and emergency air lines are crossed?

What this test is drawn from

The other CDL knowledge tests