CO ( Carbon Oxide )

Measured in percent of the total sample concentration of the exhaust gases. Associated with Partially Burned Petrol.

CO is a by-product of combustion, therefore, if combustion does not take place, carbon monoxide will not be created. Based on this premise, when a misfire occurs, the carbon monoxide that would have normally been produced during the production process is not produced. Generally speaking, on fuel injected vehicles, high CO means too much fuel is being delivered to the engine for the amount of air entering the intake manifold.

This is the petrol that has combusted, but not completely. This gas is formed in the cylinders when there is incomplete combustion and an excess of fuel. Therefore excessive CO contents are always a sign of an overly rich mixture preparation. ( The CO should have become CO2 but did not have the time or enough O2 to became real CO2 so it is exhausted as CO instead.)

CO is HIGHLY POISONOUS ODOURLESS GAS! Always work in well ventilated areas! CO combines with a chemical substance in the blood which acts as an "oxygen transporter". This substance very readily absorbs oxygen in the lungs and releases it to the body's tissues. When it combines with CO, instead of oxygen, this function is irreversibly impaired. Since CO has no smell and cannot be seen it is obviously very bad news!

1) High CO readings usually indicate a fuel mixture richer than ideal (rich mixture - air fuel ratio below 14.71). In general CO is an indicator of combustion efficiency. The amount of CO in a vehicle’s exhaust is directly related to its air-fuel ratio. High CO levels result from inadequate O2 supply needed for complete combustion. This is caused by a too rich mixture - too much fuel or not enough air (AFR readings below the optimal 14.71, Lambda below 1.0). High carbon monoxide levels are caused by anything that can make the air/mixture richer than "ideal". The following examples are typical causes of rich mixtures on vehicles:

Note:  It should be pointed out that due to the reduction ability of the catalytic converter, increases in CO emissions tend to reduce NOx emissions. It is not uncommon to repair a CO emissions failure and, as a result of another sub-system deficiency, have NOx increase sufficiently to fail a loaded-mode transient test.

Catalytic converter intervention and CO concentrations:

1) High CO readings at the tailpipe are an clear indication that there is a problem in at least one part of the system, but a CO reading that appears within "normal" ranges or is only modestly elevated is not necessarily a reliable indicator of proper or even acceptable system performance. Low range CO readings are possible, and not uncommon, from a malfunctioning engine equipped with a properly functioning catalytic converter. In such circumstances, truly elevated pre-catalytic converter CO levels will be masked by the catalytic converter and the potential for an CO problem must be further evaluated in the context of other readings of abnormal gas concentrations and AFR / Lambda readings.

2) Normal CO readings. If the combustion process is succeeding at or near the Stoichiometric point (AFR equals 14.71, Lambda equals 1.0), CO levels during an idle test will typically measure less than 2%.

3) Low CO readings. There is, effectively, no reading for CO that can be characterized as too low or "below optimal". CO concentrations will appear "normal" even in a lean burning environment, where AFR is above 14.71 (Lambda is above 1.0).


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