E5 Error Code

Hello all,
My Summit SCFF53BSS freezer has started flashing an E5 error. The door didn't get closed accidentally a week ago and the ice buildup was terrible and it took hours to defrost.
After that I plugged the unit back in to power and it was fine overnight but again this morning the E5 was flashing/beeping yet again.
I'm thinking that the defrost timer is bad, but any suggestions would be gratefully accepted.
Regards,
Doug.