As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING APU START UP WHILE TAXING INTO THE GATE. APU FAULT LIGHT ILLUMINATED. WE DISCONTINUED THE APU START AND ATTEMPTED RESTART WHEN THE APU FAULT LIGHT WENT OUT. THIS WAS REPEATED THREE TIMES. GROUNDED - YES. PERFORMED APU BITE PROCEDURE REF B737-800 FIM 49-60 TASK 801 MAINT MESS 49-61127 FOR ECU INTERNAL FAILURE. REMOVED AND REPLACED ECU REF B737-800 AMM CH 49-61-12-00-801 & AMM CH 49-61-12-400-801 APU OPS NORMAL AT THIS TIME. NO CURRENT FAULTS.