Causes of Excessive Carbon Monoxide

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 fuel injected vehicles:

• Excessive fuel pressure at the injector(s)

• Leaking fuel injector(s)

• Ruptured fuel pressure regulator diaphragm

• Loaded/malfunctioning EVAP system (two speed idle test)

• Crankcase fuel contamination (two speed idle test)

• Plugged PCV valve or hose (two speed idle test)

• Closed loop control system incorrectly shifted rich

• False input signal to ECM

-incorrect indication of load, coolant temp., O2 content, or throttle position

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.

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