What indicates a major defect with the exhaust system?

A major defect within the exhaust system is most clearly indicated by leaking in the cab. This situation poses a significant safety risk as it can lead to harmful exhaust fumes entering the passenger compartment, resulting in health hazards for the driver and passengers due to the inhalation of toxic gases like carbon monoxide. Proper functioning of the exhaust system is crucial for directing exhaust gases safely away from the vehicle's interior. Therefore, when there is a leak inside the cab, it is a clear sign that the exhaust system is compromised and requires immediate attention.

In contrast, while the smell of exhaust outside the vehicle may indicate issues with the exhaust system, it doesn't necessarily affect the safety of the vehicle's occupants in the same direct way that a leak in the cab does. A small hissing noise from the exhaust could suggest minor issues but is less serious compared to the direct health implications of a leak. Normal exhaust flow indicates that the system is functioning properly and doesn't suggest any defects at all.