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