Audi20v.net

High HC - Hydrocarbons - Indicates a misfire condition (incomplete or
 inefficient combustion).
         *  Ignition problems
         *  Carbon deposits in combustion chamber
         *  Overly lean A/F mixture
         *  Increased camshaft overlap
 
 High CO - Carbon Monoxide - Indicates partially burned fuel (not enough
 oxygen for complete combustion).
         *  Overly rich A/F mixture
         *  Dirty air filter
         *  High fuel pressure
         *  Dirty injectors
         *  Sluggish/inoperative Oxygen sensor
         *  Dirty oil
 
 High NOx - Oxides of Nitrogen - Indicates excessively high combustion
 chamber temps (such as produced during detonation).  Typically highest
 readings found during part throttle/light load on non-detonating engine.
         *  Clogged/inoperative EGR
         *  Octane of fuel too low
         *  Compression (cranking pressure) too high
 
 Things to do before going for emissions test if vehicle is modified or has
 failed a test:
 
 *   Fresh oil change.
 *  New air filter (not K&N, you don't want the oil).
 *   Run Techron through 12 gallons of fuel
 *   Two to Three heat ranges hotter on spark plugs (for test period only).
 *   Set idle up to 300 RPM over Mfg. specified idle speed.
 *   Use higher octane fuel.
 *   No vacuum leaks.
 *   Set idle mixture slightly lean (too lean can increase NOx & HC).