measure total transformer secondary current in heat mode with just the heat pump running. do the same in emergency heat mode. the transformer should be capable of handling all those loads combined if there is a need for aux heat. the transformer's limit is approx 1.7 amps (40 va / 24 v). if the current draw is approaching that number, you need to energize each low voltage load one at a time and isolate the one drawing too much current. if the wires outside are suspect, call for heat and measure current on the common wire outside while jiggling the wire bundle. if you see a spike occur, separate the wires and find the insulation break. if a direct short occurs while performing this test, something should chatter in the unit also. a 3 amp fuse will catch a short, but if the current hovers between 1.7 and 3.0, you may still lose a transformer. that would indicate a low voltage load beginning to deteriorate and intermitent rising current. if a load is failing in such a manner, it should also get hot. sometimes a mechanical hangup in a relay can cause current to rise.