Vehicle Inspection on the CDL General Knowledge Test: What to Check and Why

What the federal inspection knowledge area covers

Federal rules list 20 general knowledge areas that every CDL applicant must know, and vehicle inspection is number 17 (49 CFR 383.111(a)(17)). The rule splits it into five parts: why periodic inspection and repair matter, what an undiscovered malfunction can do, which safety-related parts to look at, how pre-trip, en route, and post-trip inspections are done, and how to report what you find. It also names the parts: fluid leaks, anything that blocks visibility, bad tires, wheel and rim defects, defects in the brakes, steering, suspension, exhaust, and coupling system, and cargo problems.

This page covers what the knowledge test can ask. The skills test has its own inspection segment, in which you point to parts and explain them to an examiner (383.113(a)), and the CDL skills test guide explains it. State manuals must be comparable to AAMVA's Model Commercial Driver Manual (383.131(a)), and the manual figures below come from Section 2.1 of Washington's CDL guide, a public copy of that model. Study from your own state's manual.

Pre-trip, en route, and post-trip inspections compared

The federal rules in this table come from the eCFR, current as of October 26, 2026.

The work is split. You inspect, report, and review. The motor carrier repairs any listed safety defect and, before the vehicle runs again, certifies on the report that it was repaired or that no repair was needed (396.11(a)(3)). The report may be electronic (396.11(a)(6)), a point FMCSA spelled out in a rule effective March 23, 2026, and a carrier that runs only one commercial vehicle is outside the report rule (396.11(a)(5)). Inspection questions planned for full CDL prep tie each explanation to the federal rule or manual figure behind it, so you learn which part of the table a question is testing.

What a defect looks like, system by system

The defects come from the federal tire rule (393.75) and Washington's guide, Section 2.1.3. The steering-play and leaf-spring figures are manual figures, not federal rules. The right-hand column is Passy's explanation.

Coupling has its own procedure, covered in the coupling and uncoupling guide. Weight, balance, and tie-downs are covered in the cargo guide. On a vehicle with air brakes, the inspection ends with the air brake check, which your manual's air brake section explains step by step.

Tread depth and other inspection numbers

Tread depth is one of the few hard numbers in this area, and it is a federal rule. Any tire on the front wheels of a bus, truck, or truck tractor needs at least 4/32 inch of tread at any point on a major tread groove (49 CFR 393.75(b)). All other tires need at least 2/32 inch in a major groove (393.75(c)). Do not measure at tie bars, humps, or fillets. A bus may not run regrooved, recapped, or retreaded tires on its front wheels (393.75(d)). The two depths are easy to swap under pressure, which is why flashcards that drill 4/32 versus 2/32 inch of tread are planned for full CDL prep.

Here is how that works on one tractor-trailer. Ana, a fictional driver, measures each tire in a major groove, away from the tie bars:

• Left steer tire, 5/32 inch: passes, because it has more than 4/32.
• Right steer tire, 4/32 inch: passes, because the rule says "at least" 4/32.
• One drive tire, 3/32 inch: passes, because tires off the front axle need only 2/32.
• One trailer tire, 1/32 inch: fails.

If that 3/32-inch drive tire were moved to the steer axle, it would fail, because the 4/32 minimum applies to any tire on the front wheels. The free CDL practice questions include a steer-tire tread item with an explanation for each option.

Emergency equipment the pre-trip must confirm

Federal rules require two kinds of emergency equipment on every truck, truck tractor, and bus (49 CFR 393.95):

• A fire extinguisher rated 5 B:C or more, or two rated 4 B:C each, and 10 B:C on a power unit hauling placarded hazardous materials. It must show its rating and whether it is charged, and be mounted securely within reach.
• Warning devices: three bidirectional reflective triangles, or at least six fusees. Flame-producing devices are banned on some explosives loads, on cargo tanks for flammable liquids or gas, and on vehicles fueled by compressed gas.

You place the warning devices within 10 minutes of stopping on the road or shoulder (392.22), and the General Knowledge test guide shows where they go.

Some manuals list more. Washington's also names spare electrical fuses (unless the vehicle has circuit breakers) and lists three liquid-burning flares as an option. The current 393.95 has neither: FMCSA removed the spare-fuse requirement and the flare option in final rules published February 19, 2026, effective April 20 and March 23, 2026. A manual printed before those rules can still show the old list, so learn your manual's version for the test and carry what current rules and your carrier require.

How the modernized skills test changed inspections, and what it did not

AAMVA publishes a current and a modernized model manual. The modernized one rewrote the skills-test chapters on vehicle inspection and basic control, and each state chooses which to use; AAMVA says there is no mandate to switch. Washington, for example, says it implemented the modernized test and replaced Sections 11 and 12 of its CDL guide with a supplemental guide, as checked on October 28, 2026. The change affects how the skills-test inspection is run and scored. The federal knowledge area in 383.111(a)(17) is the same either way, so this page applies to both, in 2026 and 2027 alike.

Studying inspection for the General Knowledge test

For each system in the defect table, learn what normal looks like, what the defect looks like, and what it can cause. Add the numbers: 4/32 and 2/32 inch of tread, the extinguisher ratings, three triangles or six fusees. Then tie each check to the inspection that catches it: a wheel or hub showing signs of heat at a fuel stop is an en route find, a defect written up at day's end is a post-trip report, and reviewing yesterday's report is a pre-trip step. If you mix those up in practice, Fix It, planned for full CDL prep, keeps each missed inspection question for another try.

Cargo timing spans all three moments: a check before you leave, one within the first 50 miles, then a recheck at each duty-status change, each 3 hours of driving, or each 150 miles, whichever comes first. The free practice set includes a flatbed recheck-timing item, so once you can recite that sequence, try it and compare your reasoning with the explanation. For the other areas, go back to the 20 federal topics.