Quote:
Originally Posted by Chaluska
High HC is caused by a incomplete burning of fuel. weak ignition (old coil / spark plugs / plug wires), leaking fuel injectors or running rich (dirty MAF / vacuum leak), the catalytic converter removes unburned HC so the cat may be worn too.. remove it and see what the mesh looks like. these can be burned all the way thru over 20 years of use.
the EGR valve wont make an effect on HC. that will cause your NOX to be sky high
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Agreed with this....I would also suggest changing the PCV Valve. This will/might reduce hydrocarbon numbers. I just recently bought about 15 feet of vacuum lines and carefully went thru and changed all the vacuums on my car and it helped make the car run better also. Some of my vacuum lines were def leaking so it caused issues upon emissions testing. If changing plugs made a big difference, you might consider changing plug wires if they are old also.