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Message: <blockquote data-quote="AaRoN" data-source="post: 561969" data-attributes="member: 8691">Elevated HC and CO tailpipe emissions are often symptoms of a fouled converter or a faulty air supply. Converters don't just plug up or die for no good reason. Prolonged overheating or short term severe overheating are the leading causes of catalytic converter plugging. The problem is often fouled or misfiring spark plugs, or a burned exhaust valve that leaks compression and allows unburned fuel to pass through the combustion chamber into the exhaust. To diagnose a plugged catalytic converter, you can check intake vacuum or exhaust backpressure. To check intake vacuum, connect a vacuum gauge to a vacuum port on the intake manifold. Start the engine and watch the vacuum reading at idle. Then increase RPM's to about 2500 and hold it there. Normal vacuum at idle for most engines should be 18 to 22 inches Hg. When was your last tune up (plugs, wires, coil packs, etc.)? Normal maintenance issues must be addressed. Also, do you have any CEL's?</blockquote>