What was found
AIRCRAFT WAS GROUNDED: FLIGHT DIVERTED TO TUL ON ACCOUNT OF APU FAULT LIGHT INDICATOR AND GENERATOR ISSUE. EMERGENCY WAS DECLARED. APU FAULT LIGHT ILLUMINATED. IAW QRH, APU WAS SHUTDOWN, AND FAULT LIGHT EXTINGUISHED AFTER ONE MINUTE. APU RE-STARTED AND GEN WENT BACK ON-LINE. FAULTED AGAIN AFTER 5-10 MINUTES. NO RE-START ATTEMPTED. MAINTENANCE FOUND APU OIL COOLER FINS CONTAMINATED. REPLACED COOLER IAW AMM 49-91-41. OPS CHECK AND LEAK CHECKED OK.