What was found
R AC BUS CAUTION MESSAGE ILLUMINATED DURING FLIGHT NO MASTER CAUTION ASSOCIATED REMAINED ILLUMINATED UNTIL ENGINE SHUTDOWN. DIVERTED. REMOVED FROM SERVICE. BOARDED ACFT TO VERIFY FAULT. NO R AC BUS CAUTION ILLUMINATED. PERFORMED FIM 24-20-00-810-806 STEPS 1-4A(2) & FIM 24-20-0-810-808 STEPS 1-4A(2) CAUTION DID NOT ILLUMINATE COULD NOT DUPLICATE FAULT. PER MTX CNTL. R&R #2 AC GCU IAW Q400 AMM 24-21-06-000-801 & 24-21-06-400-801. OPS TEST GOOD. NO FAULTS IN EPCU. #2 AC GCU R&R'D AS A PRECAUTION. AIRCRAFT RETURNED TO SERVICE.