2017 Trane Callback. Several previous visits. Heat would not come on. Found thermostat would blank out and come back on when there is a demand for heat. Checked system and had power. Fuse was not blown. Found contactor in air handler with some burns on the coil. Replaced contactor. System started and ran for around 30 minutes. System went into a defrost cycle during visit. Tested heat strips. System appears to be working properly.