Your Geo Metro may have failed the smog test because of a malfunctioning EGR
valve. The EGR valve is responsible for re-circulating a small amount of
exhaust gas back into the combustion chamber. This is done to cool down the
chamber temperature by producing an inert gas along with the regular fuel
mixture. Nitrous Oxides emissions are formed when engine chamber
temperatures rise above 2500* F. Keeping the chamber cool reduces NO
emission.
The EGR system is prone to collecting carbon build-up. Since the EGR valve
pulls exhaust gasses from your exhaust and introduces them back into your
engine's intake air over time it will become clogged or stuck open/closed
from the carbon deposits.
Have a smog check repair station inspect your Geo's EGR system. If no
problem is found there, they should look into the fuel management system and
make sure that your Geo Metro is not running lean. Lean fuel conditions
cause high NO as well.
