All right, assuming NOx and CO2 are unavailable, I can think of a few
things off the top of my head. This is in no particular order, so do the
easy, cheap stuff first.
Remember that HC is simply raw gas that's getting out of the combustion
chamber without having been burnt up first. CO is incomplete combustion,
from too much fuel and not enough oxygen (the resulting combusted
molecule has only one oxygen atom instead of two).
1) Check the cat inlet and outlet temperatures.
Take the car out for a drive sufficient to warm the engine up to full
operating temperature. A bit of high-speed driving is a good idea. When
measured with an infrared thermometer, the cat should have an outlet
about 30-100 F higher than the inlet. If in and out are the same, or the
outlet is cooler, the cat is either insufficiently warmed up, or it's no
longer working anymore.
2) Do you have EGR?
Partially-plugged EGR can cause across-the-board high emissions, which
is why I asked about NOx.
3) Retarded ignition timing.
You'd be dumping raw gas directly into the exhaust manifold, expecting
the cat to do the clean up, something it's not designed to do.
4) Dirty injectors with poor spray pattern.
Run a Motorvac service (~$100), which is not a bad idea even if there's
no actual issue to solve.
5) Thermostat too old and stuck open.
Will cause rich running and high emissions.
6) What do the spark plugs look like? What kind and color of deposits on
them? Do they show signs of proper spark? Have you checked each plug
wire to make sure you get a purply-blue spark at each one? (Use a spare
spark plug to check in a dark garage or at night.)
7) How old is the oxygen sensor? Is it OEM or aftermarket?
An old sensor will result in poor control of fuel mixture.
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