CDL Air Brake Restrictions (L and Z) and the Other Restriction Codes

What the L and Z codes mean on a CDL

Federal rules fix the letters a state uses for restrictions on a CDL (49 CFR 383.153(a)(10)). Two of them concern brakes:

• L: no air brake equipped CMV. For this restriction, "air brakes" includes any system that works fully or partly on the air brake principle (383.95(a)(2)). With an L, you may not drive an air-over-hydraulic truck either.
• Z: no full air brake equipped CMV. With a Z, you may drive a commercial vehicle with air-over-hydraulic brakes, but not one whose brakes work fully on air (383.95(b)).

If a training handout or a summary page words L or Z differently, go by the regulation, quoted above from the eCFR as current on October 26, 2026: L covers air brakes of any kind, including air over hydraulic, and Z covers only full air brakes. No change to these codes has been published or proposed for 2027.

How each federal restriction code is triggered

Most of these codes come from the vehicle you test in, not from a test you fail. Only L can come from a knowledge test score. Z, E, and O apply only to applicants who took their skills tests on or after July 8, 2015. Drivers who already held a CDL before then are not retested for them (383.153(g)).

CLP restriction codes, and letters that change meaning

A commercial learner's permit has its own list of restriction codes (383.153(b)(2)(ix)):

• P: no passengers in a CMV bus.
• X: no cargo in a CMV tank vehicle.
• L: no air brake equipped CMV.
• V: medical variance.
• M: no Class A passenger vehicle.
• N: no Class A and B passenger vehicle.
• K: intrastate only.
• Any added restriction from the state that applies to the permit.

Three letters do double duty, which makes them easy to mix up.

Read where the letter sits on the document. The endorsement and restriction fields are separate. The endorsements guide covers what each endorsement allows, and at launch full CDL prep covers the endorsement tests too, from Tank Vehicles to School Bus.

Removing L, Z, E, and O: two different paths

L, Z, and E. You do not have to retake the complete skills test. The state may give a modified skills test that shows you can safely run the vehicle's full air brakes, air-over-hydraulic brakes, or manual transmission. To remove L or Z, you must also perform the air brake pre-trip inspection and pass the Air Brakes knowledge test (383.135(b)(7)). Washington, for example, lists removing an air brake restriction among the reasons you must take a knowledge test (as checked on October 28, 2026, per Washington DOL). Entry-level driver training is not required for a restriction removal under 383.135(b)(7) (49 CFR 380.603(a)(4)).

O. There is no shortcut. You must retake all three skills tests in a representative tractor-trailer (383.135(b)(8)).

The federal text does not describe a removal path for M or N, so ask your state licensing agency how it handles a passenger restriction.

Choosing a test vehicle so you do not pick up a restriction

Because most codes come from the test vehicle, taking the skills test in the type of vehicle you will drive is the simplest way to avoid them. A worked example shows why.

Marisol, a fictional applicant, passed General Knowledge and Air Brakes and holds a Class B permit. Her training school's truck has air-over-hydraulic brakes and an automatic transmission. If she takes her skills test in it, her CDL will carry Z and E. Her job offer is for a dump truck with full air brakes and a manual transmission, so she could not legally drive it. Removing both restrictions later would take a modified skills test, plus, for the Z, the air brake pre-trip inspection and the Air Brakes knowledge test. If she tests in a full-air, manual-transmission truck instead, neither restriction appears.

The L works differently, because it can come straight from the knowledge test. If you will drive anything with air brakes, passing the Air Brakes knowledge test is the step to secure first. Air Brakes practice planned for full CDL prep gives that test a question set of its own. The free 20-question CDL set already mixes six Air Brakes items with General Knowledge and Combination Vehicles items and shows a breakdown by test at the end: check your Air Brakes answers in the free 20-question CDL set, then go back to the Air Brakes test guide for any topic you missed.

States can add their own restriction codes

The federal list is a minimum. A state may add codes for other restrictions as long as each one is explained on the front or back of the license (383.153(a)(10)(ix)), and a CLP may carry any added state restriction that applies to the permit (383.153(b)(2)(ix)(H)). Codes like that vary by state, so read the code key on your own license or permit.

To see how restrictions fit with the classes and tests, go to the tests-by-class guide. To see how the skills test vehicle is chosen, go to the CDL skills test guide. To plan around the tests your state still requires, check the question and flashcard counts planned for each CDL test.