Hi All, I have a Lennox ml195 gas furnace that's about 4 years old it was installed originally with a 2 wire turn dial thermostat which I changed out for a basic digital thermostat and that worked fine as it was battery operated. I purchased a Honeywell wifi thermostat last week and had a new 18/5 wire ran to the furnace when I connected the thermostat and furnace the onboard fuse blew out. So contacting an HVAC 24-hour emergency service they said to take off the Y and G wires as I don't have A/C or humidifier it's a heat only system. So removing those and replacing the fuse same issue. furnace runs fine with just the R and W wires connected but as soon as I connect the C wire it pops the fuse.

The issue is not the thermostat as I returned it and got a replacement thinking that was the issue. Any chance it could be caused by a faulty ground somewhere in the furnace?