I just picked up a nice, really clean 1992 Honda Accord wagon for
$400.00. It has been well-maintained, and has 224,000 miles (long
commute).

Here are the readings from the previous smog check on 05/31/05, when it
easily passed:

(Odometer 199,770)

RPM  %C02  %02  HC (PPM)  CO (%)

717  14.8  0.7  16        0.01

2695 15.2  0.0  15        0.13

As you can see, it was running pretty clean for such a high-milage car.

When it was taken for smog check on 3/17/07, it failed HC at idle:

(Odometer 223,879)
 
RPM  %C02  %02  HC (PPM)       CO (%)

699  8.2   9.4  131 (max 120)  0.40

2480 14.2  1.1  16             0.09

I also noticed that the CO (at idle) was much higher than before, but
still well-within the max of 1.0 percent. I spent several hours reading
archived posts from this newsgroup, and have found a lot of great info.
This one has me stumped, though. What could cause higher HC at idle only,
and also elevated CO readings (again at idle only)? Also, note the
differences in CO2 and 02 readings.

Looking at the repair records, I'm pretty sure this car is running the
original oxygen sensor and cat. I will probably replace the sensor as a
matter of course, but as money is a bit tight at the moment, would rather
avoid replacing the cat if possible - at least right away.

Engine runs great with no missing, plug wires and spark plugs are fairly
new, cap and rotor are 2 years old and have 27k, so I will replace those.
I didn't see that the EGR valve was ever serviced/cleaned, so will do it.
Oil was changed 1000 miles ago, but car has been mostly sitting for 7
months (probably should change, eh?)

Obviously I would like to get the HC and CO back down to normal. Still,
I'm curious if simply replacing cap and rotor and changing the oil might
get the HC from 131 PPM to the allowable max of 120 at idle.

Also, I like the idea (gleaned here) of running the car at 2000 RPM for
two minutes right before test to heat up the cat (having it "lit up").

Any help or info is always much appreciated!

Pigeon