What was found
APU FAULT LIGHT ILLUMINATED DURING APU START ON TAXI-IN. ACFT GROUNDED. REMOVED THE APU FUEL CONTROL UNIT IAW AMM 49-31-11-000-801. AFTER FURTHER INVESTIGATION IT WAS FOUND TO HAVE BEEN REPLACED ON DEFECT TSR908420. REINSTALLED THE APU FUEL CONTROL UNIT, REF B737-800 AMM 49-31-11-400-801. APU FCU LEAK AND OPS CHECK GOOD IAW FIM 49-31 TASK 808. WHILE TROUBLESHOOTING FOUND THE APU SUPPLY LINE AT THE FIREWALL INSIDE THE APU COMPARTMENT TO NOT BE SUFFICIENTLY TIGHT. TIGHTENED THE FUEL SUPPLY LINE IAW AMM 49-11-00-400-803 STEP 7L(6)(I).