Customer had a broken furnace on a Saturday night. Found the circuit board to be bad. It was not sending enough voltage to the gas valve to ignite. Also found multiple cracks in the heat exchanger so I red tagged it and shut it down after showing the home owner the issue. He also stated the night before his carbon monoxide detectors went off in the middle of the night. Good thing we found this before it was seriously dangerous. Left them with space heaters to heat the home till issue is fixed.