Check engine light?

So for the last couple of days my check engine light has been on, but last week the light came on for about 5 hours then went away but this time its been on for a while. its a 2000 XJ, i recently got a flowmaster 40 cat-back system done, could that have anything to do with the light? also the engine is full of mud like crazy, should i clean the engine and then see if it goes off?

Go get the code read so you know what it is. Could be any number of things.

just drive it until it blows lol

the downstream 02 sensor might have gotten cooked during the welding. I've seen this happen before. Need the error codes to be sure.