HVACopediaAll brands · Furnace troubleshooting
CO in flue / poor combustion numbers
What is going on
Low CO2 + high O2 + CO → excess air too high or manifold pressure low; high CO2 + low O2 + CO → air shortage or manifold pressure high.
Check in this order
- Verify manifold pressure (3.5 NG / 10.5 LP), then air, vent, HX
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Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.