What are the possible problems that this indicator could be sensing or is it likely that it's just a sensor?
Emission Light on Control panel on 2002 Ford Excursion
There are hundreds of reasons the emission light can come on, it is crucial to have the codes checked and a proper diagnosis performed. continuing to drive the vehicle with the fault could cause more damage so try to take care of it sooner than later. If you get the codes, post them here and I may be able to assist you further. good luck!