I'm sure this has been brought up many times but new to the forum and need help with this code.  Just pulled off the freeway after a 4 hour drive pulling an empty trailer, stopped to  eat  and when I started up the CEL code popped up.   Went to a local parts store read the code cleared it but it came back on after a short time.  DEF fluid has been  in the DEF tank  for about two months ands reads half a  tank.  Can old DEF fluid add to this problem with the code.  Thanks