Causes of Excessive Exhaust Emissions

As a general rule, excessive HC, CO, and NOx levels are most often caused by the following conditions:

• Excessive HC results from ignition misfire or misfire due to excessively lean or rich air/fuel mixtures

• Excessive CO results from rich air/fuel mixtures

• Excessive NOx results from excessive combustion temperatures

There are lesser known causes to each of these emissions that will be discussed later. When troubleshooting these types of emissions failures, you will be focusing on identifying the cause of the conditions described above. For example, to troubleshoot the cause of excessive CO emissions, you need to check all possible causes of too much fuel or too little air (rich air fuel/ratio). The following lists of causes will help familiarize you with the sub-systems most often related to excessive CO, HC and NOx production.

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Do It Yourself Car Diagnosis

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