Which fluid level is only checked while the engine is running?

The transmission fluid level is typically checked while the engine is running to ensure an accurate reading. This is necessary because transmission fluid expands as it heats up, and the correct fluid level can only be determined under operational conditions. When the engine runs, the transmission fluid circulates through the system, allowing the dipstick, if equipped, to provide a proper measurement of fluid level against specific markings for hot or cold situations.

Checking the fluid level while the engine is off could lead to an inaccurate measurement, as the fluid may not be properly distributed within the transmission when it is not in operation. This practice is important for maintaining optimal transmission performance and ensuring the longevity of the system, as the right fluid level is essential for adequate lubrication and cooling.