The service engine light can mean anything. Asking what it means without properly diagnosing the vehicle is like asking what cars the Ford brand makes/owns. To list all those individual things would take forever, and experimenting with those things is gonna cost more then the proper diagnosis and repair to begin with.
From what you've described, it sounds like a sensor is going. Now, depending on when you purchased your 2001 F150 and how many kilometers are on it, you should still be covered by Ford's 3 year comprehensive and powertrain warranty. If you purchased an extended service plan/extended warranty, that's even better.
The best thing to do if you have some form of the warranty intact, is to take it to the dealership and have them diagnose it and fix it.
How to tell if your warranty is in take: It runs for 3 years or 60,000km from the day of delivery. So if you picked up the truck on a that has yet to happen (say September of this year) and you have less then 60,000km on the truck, then it'll be under warranty and you have nothing to worry about.
On that note, if the problem is a wear and tear item, it is not covered in this portion of the standard warranty from Ford.
I hope this helps you.
