Question: I am in the middle of changing my transmission filter and upon removing the pan I found a plug with an o-ring around the middle of the plug. What is it and where does it go? It is a 1999 Ford F150 5.4 engine. Answer: I can tell that this is the first time the filter has been changed. The plug is installed by the manufacturer of the transmission for shipping. Once the transmission is installed on the assembly line a transmission dipstick tube is installed by shoving the plug the rest of the way through. It will not hurt anything to leave it in there, but you might as well toss it while you in there.