changing the transmission filter on my 2003 mercury grand marquis and a little plastic yellow piece with an o-ring on it fell out . What is this part and is it important and where does it go?

It s a factory piece only used in production. It serves no purpose now. -- throw it away and carry on!

Thank you a whole lot. That helps and people like you make things so much easier. Do you have any idea what purpose this thing served?

Can anyone tell me why my parking lights won't work? Same car. My husband also wants to know how he can dry fill the transmission?

That little plug is a shipping plug inserted in the dipstick tube to keep dirt out. When the transmission is installed and filled with fluid, the plug is pushed down into the pan. It won't hurt anything and it tells you by finding it, the transmission has it original fluid.

Ok..thanks for these comments..I found same thing on my 99 grand marquis..I almost fell over!! Lol