Combination Vehicles Practice Test

Required for Class A. Coupling and uncoupling, trailer brakes, off-tracking, and the handling differences that come with an articulated vehicle.

This is a core knowledge test — every CDL applicant takes it, whatever the class.

Do you need this test?

Required for Class A. Everyone takes the general knowledge test as well.

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Study the material, not just the questions

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Sample Combination Vehicles questions

1. Before backing under the trailer, make sure the:

   • A. Trailer brakes are released
   • B. Trailer is empty so the kingpin sits lower
   • C. Fifth wheel is unlocked
   • D. Trailer brakes are locked and/or wheels are chocked
   Show answer

   D. Trailer brakes are locked and/or wheels are chocked — The trailer brakes should be locked (or wheels chocked) to prevent the trailer from moving when backing under.

   Universal rule · checked against FMCSA Commercial Driver License Manual — Section 6: Combination Vehicles · verified 2026-05-17

2. The trailer service line is typically colored:

   • A. Red
   • B. Blue
   • C. Green
   • D. Yellow
   Show answer

   B. Blue — The service line (blue) carries air controlled by the foot brake or hand valve. The emergency/supply line is red.

   Universal rule · checked against FMCSA Commercial Driver License Manual — Section 6: Combination Vehicles · verified 2026-05-17

3. The trailer emergency line is typically colored:

   • A. Green
   • B. Blue
   • C. Red
   • D. Black
   Show answer

   C. Red — The emergency line (red) supplies air to the trailer air tank and controls the emergency brakes.

   Universal rule · checked against FMCSA Commercial Driver License Manual — Section 6: Combination Vehicles · verified 2026-05-17

4. Off-tracking causes the trailer to:

   • A. Brake more efficiently
   • B. Increase speed on curves
   • C. Follow a different path than the tractor when turning
   • D. Use more fuel than the tractor alone
   Show answer

   C. Follow a different path than the tractor when turning — Off-tracking means the rear wheels of a trailer follow a shorter path than the front wheels when turning. The longer the vehicle, the more off-tracking.

   Universal rule · checked against FMCSA Commercial Driver License Manual — Section 6: Combination Vehicles · verified 2026-05-17

5. When connecting the glad hands, clean seals help to:

   • A. Prevent air leaks
   • B. Improve fuel economy
   • C. Extend tire life
   • D. Reduce noise
   Show answer

   A. Prevent air leaks — Clean, undamaged seals on glad hands are essential to prevent air leaks in the brake system.

   Universal rule · checked against FMCSA Commercial Driver License Manual — Section 6: Combination Vehicles · verified 2026-05-17

Frequently asked questions

How many questions are on the Combination Vehicles test?

20 multiple-choice questions, and you need 80% correct — so you can miss 4. The federal standard is consistent across nearly every state.

Is this Combination Vehicles practice test free?

Yes. Every practice test here and everything in the DMV Prep app is free — no ads, no paywall, no account.

Where do these questions come from?

The FMCSA Commercial Driver's License Manual and 49 CFR. Every question carries its citation, which you can open from the answer explanation.