What is commonly checked during a routine engine tune-up?

During a routine engine tune-up, the condition of the spark plugs is commonly checked because they play a crucial role in the engine's performance. Spark plugs ignite the air-fuel mixture in the combustion chamber, and their wear or fouling can lead to poor engine performance, reduced fuel efficiency, and increased emissions. Therefore, inspecting and potentially replacing spark plugs is a key part of maintaining an engine's efficiency and ensuring smooth operation.

While checking the condition of brake pads, tire tread depth, and transmission fluid level is important for overall vehicle maintenance, these items do not fall under the specific focus of a routine engine tune-up, which is primarily aimed at optimizing engine performance and efficiency.