hi...just bought a nissan sentra 2011.....i'm not having any problems but would like to know how i would know when to add transmission fluid...the car has 131,000 miles

On an automatic let the engine run until is warm or check it after driving while it is warm and park on level ground and leave the engine running in park and pop the hood and look for the transmission dip stick and pull it out and take a paper towel and clean off the trans fluid then put the dip stick back in all the way and pull it out all in one motion, look at the fluid level and dip stick marks to see where it is at , it needs to be between checkered area above add a quart line, do not over fill which is above the checkered area. Transmission fluid expands when hot, add a little at a time and allow it to warm up and check level, if its at or below the add a quart line then you are safe to add a full quart.

If yours is the CVT Trans. The dipstick is to the right of the oil dipstick and has a cap. There a small latch, you'll see it. Take a small flat head, push it in while pulling up and the dipstick will come out.

Here is youtube video::::Copy & Paste it::::Nissan CVT Fluid Change with Basic Hand Tools.