3.0 s type jag with 50000 miles only

just replaced the auxiliary belt which was easy. Put everything back together and now it’s misfiring at idle and acceleration and now the engine light has come on. 

Ran fine before this, I checked everything and all is back as it should be. All that was removed initially was the air box pipe. 

Just to update (I’d forgotten about it)

Nothing to do with the belt. It was a wet spark plug hole, just coincidence it appeared after I did the belt. It wa s the common fault of rain water leaking in to the hole.