Which of the following is NOT a key component of the engine that should be checked during a pre-trip inspection?

During a pre-trip inspection, it is essential to focus on components that ensure the engine's proper functioning and overall vehicle safety. The oil level, coolant level, and transmission fluid level are all critical to the engine's performance. They play a significant role in lubrication, cooling, and overall operational efficiency.

On the other hand, brake pad thickness, while important for vehicle safety, pertains to the braking system rather than the engine itself. During a pre-trip inspection, the emphasis is placed on inspecting components directly related to engine operation and performance, which is why brake pad thickness does not fall under the category of engine checks. This distinction helps drivers ensure that both the engine and braking systems are adequately maintained, promoting safe driving practices.