Do you check your transmission fluid with the car running

Check the Level With the engine warmed up, leave the car idling in park on a level surface. Pull out the dipstick, wipe it clean, replace it slowly, and then pull it back out. Check the fluid level—how high the fluid comes up on the dipstick—against the "full" and "low" or "fill" marks on the dipstick.

Your car's engine should be running when you add fluid to the transmission, but you should have the transmission in park and the handbrake activated for safety. Check the owner's manual for instructions on selecting the correct transmission fluid for your vehicle.

It is better to check a car's transmission fluid when the engine is running. This is because transmission fluid expands and diminishes when it is cold. If your car's fluid is checked when it is running warm, you can easily tell its amount. But when cold, the fluid may appear low, and this may lead to overfilling.