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Possible Causes of High Carbon Monoxide (CO) Emissions
High CO means too much fuel. Fuel can only come from three sources: the crankcase vapor control system, the evaporative control system, or the actual fuel delivery system.
The fuel delivery system is certainly the most likely culprit but in most cases it is very quick and easy to eliminate the other two possibilities first. However, if it is problematic to check the evaporative system purging, it is probably wise to go straight to fuel delivery system diagnosis.
In order to pinpoint the cause of excessive CO emissions, the following systems and possible defects will need to be checked, usually (but not always) in this order:
Excessive Crankcase Blowby or PCV Flow
If oil vapours in the engine's crankcase are excessive, or the oil is dirty or contaminated, excessive CO emissions can result. Dirty or contaminated oil is easily rectified by an oil change. Excess crankcase vapour flow can be caused by an incorrect PCV valve or by serious internal engine damage such as worn-out piston rings. Serious internal engine damage can only be remedied by major engine repair or overhaul.
For more info regarding the diagnosis of crankcase vapour control systems, see Crankcase Vapour Control System Testing.
Saturated Evaporative Control System
The charcoal canister stores fuel system vapors until they can be withdrawn and burned in the engine. Under normal circumstances, this should never result in excessive CO for more than a few seconds. It is possible for charcoal canisters to become saturated with fuel. This essentially means that they can never be cleared of HC vapors and need to be replaced.
For more info on how testing for excessive crankcase blow-by gases, see Evaporative Control System Testing.
Rich Air-Fuel Mixture
There are many aspects of the engine's fuel delivery system that may need to be checked when diagnosing a rich air/fuel mixture. That is the main reason that this is listed behind crankcase vapor and evaporative emission control systems-even though they are less likely causes of the problem, they are much easier and quicker to check.
When diagnosing a rich air-fuel mixture, it is important to keep in mind the possibility of a defect in an entirely different area affecting the operation of the fuel delivery system. For example, certain mechanical engine defects may cause abnormally low engine vacuum and result in the fuel system delivering more fuel than is actually needed by the engine. The symptom of excess CO may lead one to think that the defect lies with the fuel system but the problem actually lies elsewhere.
For detailed info regarding the diagnosis of fuel delivery systems, see Fuel System Testing.
Are Other Problems Being Masked?
An additional consideration if your vehicle failed with very high CO emissions is the possibility that it may also have a NOx problem that was masked by the high CO. Sometimes restoring the air/fuel ratio to the proper range uncovers a NOx problem. The NOx defect may have existed previously but wasn't evident because of the lack of oxygen that accompanies a rich mixture.
If possible, the technician should perform a quick check of NOx control systems after identifying the cause of the CO problem. Any problems found should be included in the total repair estimate. However, in these circumstances any NOx related repairs would be a lower priority than the CO related repairs.
At any rate, you should be aware that it is possible that a NOx problem may become evident after a CO problem is repaired.
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