Performed 1 system heating diagnostics on the mainfloor Trane furnace. The furnace is a 2002 model and is in poor condition. I found that the thermostat was calling for heat but the board in the furnace was setting there idle. I ran the unit several times and the furnace ran with no issues; I couldn’t get the furnace to fail again so I couldn’t determine if the board was intermittently failing or the thermostat was intermittently failing. I advised the homeowner of the potential failing furnace board and/or thermostat. I used my inspection camera to look at the heat exchangers and I did find a potential crack. I ran the furnace and found that the furnace was leaking low levels of Carbon Monoxide into the home. I advised the homeowner about the furnace is leaking low levels of CO into the home approx 1-3 ppm. The furnace is was not leaking enough CO at the time of service call but is a safety concern. The furnace may potentially leak higher and more dangerous levels sometime in the fut