CO too high = Car runs rich, burns too much gas.Bad carburetor adjustment, float too high, carburetor flooding, sticking secondary throttle, leaking fuel injector, leaking fuel pressure regulator, feedback system malfunction oxygen sensor, coolant sensor, throttle position sensor, emissions vapor system malfunction purge valve, canister clogged Be aware that NOX will RISE after you fix a CO problem. This must be checked after a successful CO repair.
HC too high = unburned fuel in exhaust; car runs very rich or very lean. This is the trickiest condition because there are different causes. If the car is running rich high CO, the HC can be high also. If the CO is very low and HC is high then the vehicle has a lean misfire.
If The CO is high, fix that problem first. If CO is low and HC is high look for engine skipping, misfire due to vacuum leaks, ignition timing set too advanced, ignition misfire problem plugs, wires, cap, low fuel pressure, carburetor adjusted too lean. If you can’t find a reason for the high HC then perform balance test then compression test to check for engine mechanical problem.
NOX too high = NOX forms in normal combustion. NOX too high indicates high combustion chamber temperatures or malfunctioning post-combustion emission controls.
CO and HC must be normal before attempting to repair NOX failure. Anything that can create high combustion chamber temperatures will make NOX reading high. Engine overheating, air cleaner hot air door stuck closed, egr vale not functioning, clogged egr valve passage, lean fuel mixture, ignition timing set too high. The two main systems that reduce NOX on a car are the egr system and catalytic converter. If your car is pinging, the NOX will be too high. Pinging is a symptom of high combustion chamber temperature. If all the test readings are just a little too high slightly high CO, HC, and NOX it is likely the catalytic converter is damaged or empty. If your converter is supposed to have air injection and it is disabled, the converter will not function properly.
via Emissions Test.