Customer had a broken Lennox furnace on a Saturday night. Found the circuit board to be failed and not sending enough voltage to the gas valve. Also found multiple cracks in the heat exchanger so red tagged the furnace after showing the owner the issue. He informed me that his carbon monoxide detectors went off the night before so it’s a good thing we found that. Customer is going to replace the furnace instead of repairing it so he doesn’t have to worry about it anymore. It was 25 years old.