the recent post about the 'German Auto' mechanic testing tune-up CO at the
tailpipe infuriated me, so here goes...

when doing a *tune-up*, the CO level must be checked PRE-catalyst.
catalytic converters are designed to reduce an excess of CO to CO2 and
water.  measuring the CO level POST-catalyst for a tune-up tells you only
that the converter is converting some unknown CO level to an acceptable
spec.  the engine *may* be in spec, but it also may be running a high CO
with attenuation by the converter.

when doing *emissions checks*, the CO level should be taken POST-catalyst to
measure the actual emissions from the tailpipe.  your car has a converter
and your emissions test should reflect values obtained after it has done its
job.

while those sampling ports do break, a mechanic armed with a torch and some
penetrating oil can usually unscrew them.  as the auto owner, occasionally
remove that port cover and coat the threads with high-temp anti-sieze so
that it WON'T break.

-dan