Vehicle Systems and Malfunctions
What this domain tests
Vehicle Systems and Malfunctions tests whether you recognize warning signs and know when to stop. The exam covers engine systems, air pressure, brakes, tires, electrical components, and coupling. The key idea is that a warning is information, not an inconvenience; ignoring it can turn a repair into a crash.
Warning lights that mean stop
A red oil-pressure warning means stop safely and shut down the engine. Low oil pressure can destroy an engine in minutes. A coolant-temperature warning means stop and let the system cool before checking. An alternator or charging warning means the battery is not being charged; you may have limited time before electrical systems fail. A red brake-system warning is also a stop condition. Do not “drive to the next exit” when the warning is red and the gauge confirms a problem.
Air and brake symptoms
Low air pressure is a critical symptom. The warning device must activate before 60 psi, and the spring brakes should apply between 20 and 45 psi. If the pressure is falling, stop in a safe place and find the leak. Do not continue and hope the compressor catches up. A brake pedal that feels spongy, a slack adjuster with too much pushrod travel, or a chamber that leaks are defects, not driving styles.
Tires, coupling, and electrical
Check tire pressure and look for cuts, bulges, and exposed cords. A thumping tire or a sudden vibration may be a tread separation. Check the fifth-wheel jaws and kingpin after coupling; a tug test is not a substitute for a visual check. Electrical problems can affect lights, turn signals, and antilock brakes, so a blown fuse is not automatically harmless.
Worked example
The oil-pressure gauge drops and the red warning light comes on at highway speed. The correct reasoning is to signal, move to the shoulder, stop, and shut down the engine. Continuing to the next exit risks seizure and a loss of steering or power. You check the oil level only after stopping; if it is low, you do not simply add oil and continue without finding the leak. The warning is the priority.
Traps that cost marks
Treating a warning light as a sensor error is dangerous. Adding coolant to a hot engine can cause burns. A low-air warning is not a suggestion to drive faster to build pressure. Do not assume a tire that looks fine is safe; sidewall damage and low pressure may not be visible.