I just replaced the aux battery and filters and oil. Check engine light is now on. Could starting up after replacing the fuel filter cause the check engine light to light up? My scanner says K57/2 relay or wire at fault. Before I replace the relay what other possibilities should I check first? I will check the fuse this morning right after I finish my coffee.
(2005 e320 cdi)

A blown fuse could have definitely triggered a CEL, did you encounter any?