Causes of Excessive Oxides of Nitrogen

Excessive oxides of nitrogen can be caused by anything that makes combustion temperatures rise. Typical causes of high combustion temperature on fuel injected vehicles include:

• Cooling system problems

- insufficient radiator airflow

- low coolant level

- poor cooling fan operation

- thermostat stuck closed or restricted

- internal radiator restriction

• Excessively lean air/fuel mixture

- leaky intake manifold gasket

- worn throttle shaft

• Closed loop control system incorrectly shifted lean

• Improper oxygen sensor operation

- slow rich to lean switch time

- rich biased 02 sensor voltage

• Improper or inefficient operation of EGR system

- restricted EGR passage

- EGR valve inoperative

- EGR modulator inoperative

- plugged E or R port in throttle body

- faulty EGR VSV operation

- leaky/misrouted EGR hoses

• Improper spark advance system operation

- incorrect base timing

- false signal input to ECM

- improper operation of knock retard system

• Carbon deposits on intake valves

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