CDL Combination Vehicles test: free practice drill and what it covers

The CDL Combination Vehicles test is the Class A knowledge test on tractor-trailers and other combinations: 20 multiple-choice questions in Washington (16 correct to pass) on driving combinations safely, the trailer air brake system, and coupling and uncoupling. The free drill below is 12 original questions with every answer explained.

20
questions on the real test: 16
correct to pass (80%): Class A
applicants take it: 4
misses is a fail

Last checked 20 October 2026 · Counts from the Washington State Department of Licensing; pass mark from 49 CFR 383.135

Free Combination Vehicles drill: 12 original CDL-style questions at a practice pace of a minute a question, then every answer explained. No sign-up.

12
questions
12 min
time limit
Free
every answer explained

By the LifeScore Editorial Team · Updated 20 October 2026 · How we check facts · How this is scored

What is on the CDL Combination Vehicles test?

Section 6 of your state manual: why combinations roll over and jackknife, off-tracking and backing, the trailer air brake system and its controls, coupling and uncoupling a tractor-trailer, and inspecting a combination.

It is the shortest of the three Class A tests, which makes it the least forgiving: with 20 questions, five misses is a fail. Most misses come from the trailer air system (which line is which, what the tractor protection valve does) and the coupling steps.

Some states give a 25-question Combination test; the content is the same. Doubles and triples are a separate endorsement test (T) with its own section of the manual, though Combination questions touch on them.

What does the test ask about driving combinations?

Rollover risk, rearward amplification, off-tracking, trailer skids and jackknifes, and how empty combinations brake.

Rollovers: fully loaded rigs are far more likely to roll over than empty ones. Keep cargo low and slow down before turns and ramps.
Rearward amplification: quick steering whips the rear trailer, so the last trailer of doubles or triples is the most likely to roll.
Off-tracking: in a turn, the rear wheels cut inside the front wheels. On right turns, keep the rear close to the curb and swing wide as you complete the turn.
Trailer jackknife: the trailer wheels lock and the trailer swings out. Release the brakes; do not use the trailer hand brake to straighten the rig.
Empty and bobtail tractors: less weight means less traction, so they take longer to stop and skid more easily.

How does the trailer air brake system work?

Two air lines connect tractor and trailer: the emergency (supply) line, usually red, fills the trailer tanks and controls the trailer emergency brakes; the service (control) line, usually blue, carries the braking signal. The tractor protection valve keeps the tractor's air if the trailer side fails.

Trailer air controls and parts, from the state CDL manual
Part	What it does
Trailer air supply control	Red, eight-sided knob. In: supplies air to the trailer. Out: shuts it off and applies the trailer emergency brakes
Tractor protection valve	Closes automatically when pressure falls to between 20 and 45 psi, protecting the tractor's air
Emergency (supply) line	Usually red. Fills the trailer tanks; losing pressure in it sets the trailer emergency brakes
Service (control) line	Usually blue. Carries the signal from the foot brake or trailer hand valve
Glad hands	Couplers that join the air lines; check the rubber seals and keep them clean
Trailer hand valve	Applies only the trailer brakes. For testing them, never for parking
Shut-off valves	Open on every trailer that tows another; closed at the rear of the last trailer

Crossed air lines are a classic question. On a trailer with spring brakes, the brakes will not release, which warns you. On an older trailer without spring brakes, you could drive away with no trailer brakes at all, which is why you check the connections every time.

What are the steps for coupling a tractor-trailer?

Inspect the fifth wheel and the area, chock the trailer, line up and back slowly under the trailer, connect the air lines and supply air, finish backing until it locks, then test the coupling with a tug and check the fifth wheel by eye before raising the landing gear.

Inspect the fifth wheel: greased as needed, tilted down at the rear, jaws open. Check the area and chock the trailer wheels.
Line the tractor up directly in front of the trailer, with the trailer slightly lower than the center of the fifth wheel.
Back slowly until the fifth wheel just touches the trailer, then secure the tractor.
Connect the air lines (emergency and service), supply air to the trailer, and check the trailer brakes are on.
Back slowly until the fifth wheel locks. Raise the landing gear slightly and pull gently forward in low gear to test the coupling.
Look at the coupling: no gap between the upper and lower fifth wheel, jaws closed around the kingpin shank, release arm locked with the safety latch over it.
Connect the electrical cord, raise the landing gear fully, remove the chocks and check the lights before driving.

Uncoupling runs the steps in reverse: park on firm, level ground in a straight line, ease pressure on the locking jaws, set the brakes, lower the landing gear, disconnect and secure the lines, pull the release, and pull the tractor partly clear to check the trailer is stable before driving away.

What do CDL Combination questions look like?

Questions on the trailer air system, coupling and driving a combination, with four options. Three examples follow; the first is worked in full and the others open when you tap.

CDL-style question 1 · Air system
What happens when you pull out the red, eight-sided trailer air supply knob?
1. AThe trailer brakes release
2. BThe air to the trailer is shut off and the trailer emergency brakes come on
3. CThe fifth wheel unlocks
4. DThe tractor parking brakes release
Worked answer
B: The air to the trailer is shut off and the trailer emergency brakes come on. Pulling the red knob closes the tractor protection valve, cutting air to the trailer, so the trailer's spring (emergency) brakes apply.
1. Red, eight-sided knob = trailer air supply.
2. Pulled out = supply off.
3. No supply air = trailer emergency brakes on.
The trap: Mixing it up with the yellow diamond knob, which controls the parking brakes on the tractor (and trailer, when supplied).
CDL-style question 2 · Doubles
You are coupling doubles. Trailer 1 is loaded to 22,000 lb and trailer 2 to 14,000 lb. Which goes directly behind the tractor?
1. ATrailer 2, because the lighter trailer goes first
2. BTrailer 1, because the heavier trailer goes first
3. CEither; the order does not matter
4. DTrailer 2, so the heavy one is at the back for traction
Show the worked answer
B: Trailer 1, because the heavier trailer goes first. The heavier trailer goes first, behind the tractor, and the lighter one at the rear, which keeps the rear trailer from pushing and whipping.
1. Compare the weights: 22,000 lb is heavier.
2. Rule: heaviest trailer first.
3. Trailer 1 goes behind the tractor.
The trap: Thinking the heavy trailer belongs at the back "for stability". A heavy rear trailer amplifies swing in a quick lane change.
CDL-style question 3 · Coupling
After coupling, you look at the fifth wheel. Which of these shows the coupling is right?
1. AThe jaws are closed around the head of the kingpin
2. BThere is a small gap between the upper and lower fifth wheel
3. CNo gap between the fifth wheels, the jaws closed around the shank of the kingpin, and the release arm locked
4. DThe release arm is in the open position
Show the worked answer
C: No gap between the fifth wheels, the jaws closed around the shank of the kingpin, and the release arm locked. The upper and lower fifth wheel should touch with no gap, the jaws should close around the shank (not the head) of the kingpin, and the locking lever should be in the locked position with the safety latch over it.
1. Look for a gap: there must be none.
2. Check the jaws are around the shank.
3. Check the release arm is locked and latched.
The trap: Accepting a small gap. A gap can mean the kingpin is riding on top of the jaws (a high hitch), and the trailer can come loose.

Do not let four questions cost you the permit

Four Combination drills and eight full mocks, every answer explained from the manual.

The CDL Pass Pack gives you 8 full mocks on the real clock, 12 drills of 12 questions for the sections that cost you points, and a predictor that turns your mocks into the odds of reaching your target. One payment, no subscription, and your free score stays free.

Prep courses for these tests usually cost $19 to $129. The CDL Pass Pack is US$39 (£29), once.

See the CDL Pass Pack How our questions are built and checked

8
timed mocks: 12
topic drills: 1
score predictor

How should I study for the Combination test?

Read section 6 of your state manual, learn the trailer air controls as a table (what each does when it loses air), practise the coupling steps in order, and drill until you clear 80% with a margin.

The Combination test assumes you know the Air Brakes material, so study that first: the 20 to 45 psi range shows up on both, once for the spring brakes and once for the tractor protection valve.

Because the test is short, two or three unlucky questions can sink it. Aim for 90% on drills, not 80%, before you go. The full CDL practice test scores Combination alongside the other two.

Frequently asked questions

How many questions are on the CDL Combination test?

Washington lists 20 questions with 16 correct needed. Some states ask 25.

Who needs the Combination Vehicles test?

Class A applicants, because a Class A vehicle is a combination.

Is doubles and triples part of the Combination test?

No. Doubles and triples is the separate T endorsement test, though the Combination test touches on the shut-off valves and trailer order.

What color is the emergency line?

Usually red. The service line is usually blue.

When does the tractor protection valve close?

Automatically when air pressure falls to between 20 and 45 psi, or when you pull out the trailer air supply knob.

Pass Combination with a margin

Four Combination drills, eight full CDL-style mocks and every answer explained, in one payment.