A closer look
That satisfying ratcheting noise you hear at the pump isn't just a friendly audio cue that you're done twisting. The cap contains a built-in torque-limiting clutch that prevents you from overtightening and shattering the plastic threads. More importantly, it creates a perfectly hermetic seal that traps highly volatile hydrocarbons inside the tank. If you fail to turn it until it clicks, the diagnostic system will assume a massive leak is occurring and angrily throw a check engine light directly into your face.
