As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE PERFORMING T.C. #78273000, #1 ENG HAS SHORT TIME FAULT 75-20421 THE HMU VBV CONTROL CURRENT IS OUT OF RANGE, ON MULTIPLE LEGS. GROUNDED - YES. REPLACED THE #1 ENGINE EEC PER B737-800 AMM 73-21-60/401. OP-CHECKS GOOD. FAULT NO LONGER ACTIVE.