Trouble Shooting Carbon-Monoxide-Failures (CO) will save you and your customers money at the gas pump.

Excessive Carbon Monoxide, (CO) is the result of fuel being burned without enough oxygen to allow complete combustion. When diagnosing the cause of a CO emission failure, keep in mind there are usually four areas of concern.

• Air restriction

• A sensor causing an enrichment effect

• Air/fuel adjustments

• Fuel delivery problems

Using a scan tool the proper way or using a volt meter you can observe the operation of the computer feedback system. A non response or sluggish O2 sensor can increase emissions.

You can observe the cycling of the o2 sensor while watching the cross counts or the switching performance of the o2 sensor with a voltmeter.

Reasons For High Carbon Monoxide

• Before you start using a scan tool to diagnose a vehicle for high emissions, always start with the basics.

• Fuel Pressure, confirm regulator is not stuck close or rupture or plugged fuel return line (check fuel pressure)

• A stuck open purge valve, discount the purge valve and see if the emissions drop

• Diluted or contaminated engine oil

• PCV system operation

• Fuel injector’s operating correctly, example leaking or dripping injectors

• Air restriction

• After confirming the vehicle is in close loop and in fuel control test the Catalytic Converter

Keep in mind High Co is an indication of a rich condition, and it’s a odorless colorless gas that is deadly.


Most people will try to replace a Catalytic Converter to take care of a rich condition. If Co is higher than 1.5% and o2 is low a new Catalytic Converter will meet a sudden death.

A rich running vehicle will eventually plug the Catalytic Converter to the point where it will become restricted.

If this happens the vehicle will have no power or it may not even start if it gets severe enough.

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"By Nate Davis, Copyright © 2011 - 2013 Automotive-Emissions-"