Arrived at the customer’s home to build a quote for re-running the flue vent. The PVC flue vent is ran into a metal flue vent that then runs to the roof in the attic. Performed a full system inspection while here. Tested heating from the thermostat. Measured the electrical component readings. Inspected the safety and cleanliness of the system. Detected carbon monoxide below the flue vent. Checked and replaced the 1” air filter. Reported findings to the customer. Strongly recommended leaving the heat off until the issues are addressed. Left the thermostat in the Off position when leaving.