High HC, Low or Normal CO at Idle

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

POSSIBLE CAUSE ACTION


Electrical misfire See IGNITION SYSTEM.



Lean Air-Fuel ratio Check oxygen sensor and feedback
Contaminated or defective oxygen sensor system. Refer to COMPUTERS AND CONTROL SYSTEMS.



Vacuum leak Inspect vacuum hoses for
proper routing, and damaged lines.
Check intake manifold area.



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



Idle speed too low Check Idle Speed Control (ISC) Valve