Hello,

I have thermostat set at 75F for COOL, but the heater in Train electric furnace unit also kicking at the same time.

I have replaced 90343 Relay but no help. Please advise the poor man:confused:

Could be at the thermostat, Maybe look for white wire on thermostat(sometimes heat).
And disconnect. Now if it cools, and no Heat, thermostat defective or misswired.
Heat may not be white wire. A short on the thermostat wiring can also be the culprit.