Carbon monoxide is formed during incomplete combustion when there is too much fuel or not enough oxygen. High levels of CO indicate a rich mixture and can be fatal if inhaled over a short period in high enough concentrations. High CO can be caused by issues like air intake restrictions, fuel leaks, carburetor malfunctions, exhaust leaks, or catalytic converter problems that result in a rich air-fuel mixture. Carbon dioxide is considered a greenhouse gas rather than a pollutant and normal tailpipe CO2 levels range from 13-17%, with CO2 and CO levels moving in opposite directions depending on whether the mixture is rich or lean.

1. Carbon Monoxide is formed when incomplete combustion
occurs. Normally, CO2 would be produced during
conmbustion, but if there is too much fuel (or not enough air)
there are not enough Oxygen molecules to complete the
“oxidation” of the Carbon.
High levels of CO are a clear indication of a rich mixture.
Besides causing pollution, inhaled CO can be fatal over even a
short period of time in sufficient density.

2. High CO is always caused by a rich mixture, but
the root cause may be any of the following:
• Air intake restrictions
• Unmetered fuel entering the engine (contaminated
crankcase – saturated EVAP canister)
• Carbuetor malfunctions
• Air Injection system malfunction
• Oxygen sensor faults
• Exhaust leaks before the upstream Oxygen sensor
• High fuel pressure, leaking injectors or leaking fuel
pressure regulator.
• Catalytic Converter operation

3. • Of the 5 gases measured only HC, CO and
NOX are considered pollutants. CO2 is
considered to be a “greenhouse gas”.
• CO2 is used as a combustion efficiency
indicator.
• Normal tailpipe CO2 levels range from 13-17%
• CO2 & CO should always be moving in
opposite directions.
• If CO is high then CO2 should be low.
• If CO2 is high then CO should be low.