High HC, Low or Normal CO - at Idle

This condition, at idle, indicates abnormally high Hydrocarbons (HC) while the Carbon Monoxide (CO) content remains normal or low.

POSSIBLE CAUSE ACTION

Electrical misfire Inspect ignition system.



Incorrect initial timing Check and adjust timing



Vacuum leak Inspect vacuum hoses for damage and proper routings. Check the intake manifold area.



EGR at idle Check EGR system



Leaking exhaust valves or piston rings Perform compression test and cylinder leak down test.



Idle speed too low Check operation of any idle speed controlling device. Check and adjust idle speed