Customer informed tech that low voltage fuse blew prior to previous tech coming out due to replacing focus pro thermostat with smart thermostat without turning off power. Previous tech replaced fuse. Tech arrived today to find a second focus pro thermostat sitting askew on original focus pro backplate, not making contact. Old focus pro is in a drawer. Tech installed new backplate from new focus pro under new focus pro. Making good contact. Tested 16 year old gas pack. Heat exchanger shows some corrosion. No cracks or holes. LP gas pressure is 10" WC. No further challenges. Upon testing, burners did not light after inducer started. Tech ensured low voltage plugs are secured. System lit 3 times in a row. Back heating. Tech collected trip charge.